[Concurrency] Remove redundant Sendable conformances in the standard library.

These conformances are all redundant because the `Sendable` conformance is
inherited from a superclass.

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -63,9 +63,6 @@ internal final class __EmptyArrayStorage
   }
 }
 
-@available(*, unavailable)
-extension __EmptyArrayStorage: Sendable {}
-
 #if $Embedded
 // In embedded Swift, the stdlib is a .swiftmodule only without any .o/.a files,
 // to allow consuming it by clients with different LLVM codegen setting (-mcpu
@@ -120,9 +117,6 @@ internal final class __StaticArrayStorage
   }
 }
 
-@available(*, unavailable)
-extension __StaticArrayStorage: Sendable {}
-
 /// The empty array prototype.  We use the same object for all empty
 /// `[Native]Array<Element>`s.
 @inlinable
@@ -312,9 +306,6 @@ internal final class _ContiguousArrayStorage<
   }
 }
 
-@available(*, unavailable)
-extension _ContiguousArrayStorage: Sendable {}
-
 @_alwaysEmitIntoClient
 @inline(__always)
 internal func _uncheckedUnsafeBitCast<T, U>(_ x: T, to type: U.Type) -> U {
@@ -977,9 +968,6 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
   }
 }
 
-@available(*, unavailable)
-extension _ContiguousArrayBuffer: Sendable {}
-
 /// Append the elements of `rhs` to `lhs`.
 @inlinable
 internal func += <Element, C: Collection>(

stdlib/public/core/DictionaryBridging.swift

@@ -421,9 +421,6 @@ final internal class _SwiftDeferredNSDictionary<Key: Hashable, Value>
   }
 }
 
-@available(*, unavailable)
-extension _SwiftDeferredNSDictionary: Sendable {}
-
 // NOTE: older runtimes called this struct _CocoaDictionary. The two
 // must coexist without conflicting ObjC class names from the nested
 // classes, so it was renamed. The old names must not be used in the new

stdlib/public/core/DictionaryStorage.swift

@@ -110,9 +110,6 @@ internal class __RawDictionaryStorage: __SwiftNativeNSDictionary {
   }
 }
 
-@available(*, unavailable)
-extension __RawDictionaryStorage: Sendable {}
-
 /// The storage class for the singleton empty set.
 /// The single instance of this class is created by the runtime.
 // NOTE: older runtimes called this class _EmptyDictionarySingleton.
@@ -139,9 +136,6 @@ internal class __EmptyDictionarySingleton: __RawDictionaryStorage {
 #endif
 }
 
-@available(*, unavailable)
-extension __EmptyDictionarySingleton: Sendable {}
-
 #if _runtime(_ObjC)
 extension __EmptyDictionarySingleton: _NSDictionaryCore {
   @objc(copyWithZone:)
@@ -511,6 +505,3 @@ extension _DictionaryStorage {
     return storage
   }
 }
-
-extension _DictionaryStorage: @unchecked Sendable
-  where Key: Sendable, Value: Sendable {}

stdlib/public/core/ExistentialCollection.swift

@@ -166,9 +166,6 @@ internal final class _IteratorBox<Base: IteratorProtocol>
   internal var _base: Base
 }
 
-@available(*, unavailable)
-extension _IteratorBox: Sendable {}
-
 //===--- Sequence ---------------------------------------------------------===//
 //===----------------------------------------------------------------------===//
 
@@ -402,9 +399,6 @@ internal class _AnyCollectionBox<Element>: _AnySequenceBox<Element> {
   ) -> _AnyCollectionBox<Element> { _abstract() }
 }
 
-@available(*, unavailable)
-extension _AnyCollectionBox: Sendable {}
-
 @_fixed_layout
 @usableFromInline
 internal class _AnyBidirectionalCollectionBox<Element>
@@ -468,9 +462,6 @@ internal class _AnyBidirectionalCollectionBox<Element>
   internal func _formIndex(before i: _AnyIndexBox) { _abstract() }
 }
 
-@available(*, unavailable)
-extension _AnyBidirectionalCollectionBox: Sendable {}
-
 @_fixed_layout
 @usableFromInline
 internal class _AnyRandomAccessCollectionBox<Element>
@@ -528,9 +519,6 @@ internal class _AnyRandomAccessCollectionBox<Element>
   ) -> _AnyRandomAccessCollectionBox<Element> { _abstract() }
 }
 
-@available(*, unavailable)
-extension _AnyRandomAccessCollectionBox: Sendable {}
-
 @_fixed_layout
 @usableFromInline
 internal final class _SequenceBox<S: Sequence>: _AnySequenceBox<S.Element> {
@@ -623,9 +611,6 @@ internal final class _SequenceBox<S: Sequence>: _AnySequenceBox<S.Element> {
   internal var _base: S
 }
 
-@available(*, unavailable)
-extension _SequenceBox: Sendable {}
-
 @_fixed_layout
 @usableFromInline
 internal final class _CollectionBox<S: Collection>: _AnyCollectionBox<S.Element>
@@ -820,9 +805,6 @@ internal final class _CollectionBox<S: Collection>: _AnyCollectionBox<S.Element>
   internal var _base: S
 }
 
-@available(*, unavailable)
-extension _CollectionBox: Sendable {}
-
 @_fixed_layout
 @usableFromInline
 internal final class _BidirectionalCollectionBox<S: BidirectionalCollection>
@@ -1036,9 +1018,6 @@ internal final class _BidirectionalCollectionBox<S: BidirectionalCollection>
   internal var _base: S
 }
 
-@available(*, unavailable)
-extension _BidirectionalCollectionBox: Sendable {}
-
 @_fixed_layout
 @usableFromInline
 internal final class _RandomAccessCollectionBox<S: RandomAccessCollection>
@@ -1251,9 +1230,6 @@ internal final class _RandomAccessCollectionBox<S: RandomAccessCollection>
   internal var _base: S
 }
 
-@available(*, unavailable)
-extension _RandomAccessCollectionBox: Sendable {}
-
 @usableFromInline
 @frozen
 internal struct _ClosureBasedSequence<Iterator: IteratorProtocol> {

stdlib/public/core/SetStorage.swift

@@ -105,9 +105,6 @@ internal class __RawSetStorage: __SwiftNativeNSSet {
   }
 }
 
-@available(*, unavailable)
-extension __RawSetStorage: Sendable {}
-
 /// The storage class for the singleton empty set.
 /// The single instance of this class is created by the runtime.
 // NOTE: older runtimes called this class _EmptySetSingleton. The two
@@ -130,9 +127,6 @@ internal class __EmptySetSingleton: __RawSetStorage {
 #endif
 }
 
-@available(*, unavailable)
-extension __EmptySetSingleton: Sendable {}
-
 #if $Embedded
 // In embedded Swift, the stdlib is a .swiftmodule only without any .o/.a files,
 // to allow consuming it by clients with different LLVM codegen setting (-mcpu
@@ -316,9 +310,6 @@ final internal class _SetStorage<Element: Hashable>
 #endif
 }
 
-@available(*, unavailable)
-extension _SetStorage: Sendable {}
-
 extension _SetStorage {
   @usableFromInline
   @_effects(releasenone)

stdlib/public/core/SwiftNativeNSArray.swift

@@ -61,9 +61,6 @@ internal class __SwiftNativeNSArrayWithContiguousStorage
   }
 }
 
-@available(*, unavailable)
-extension __SwiftNativeNSArrayWithContiguousStorage: Sendable {}
-
 private let NSNotFound: Int = .max
 
 // Implement the APIs required by NSArray 
@@ -325,9 +322,6 @@ extension __SwiftNativeNSArrayWithContiguousStorage {
   }
 }
 
-@available(*, unavailable)
-extension _SwiftNSMutableArray: Sendable {}
-
 /// An `NSArray` whose contiguous storage is created and filled, upon
 /// first access, by bridging the elements of a Swift `Array`.
 ///
@@ -435,9 +429,6 @@ extension _SwiftNSMutableArray: Sendable {}
   }
 }
 
-@available(*, unavailable)
-extension __SwiftDeferredNSArray: Sendable {}
-
 /// A `__SwiftDeferredNSArray` which is used for static read-only Swift Arrays.
 ///
 /// In contrast to its base class, `__SwiftDeferredStaticNSArray` is generic
@@ -593,6 +584,3 @@ internal class __ContiguousArrayStorageBase
       self !== _emptyArrayStorage, "Deallocating empty array storage?!")
   }
 }
-
-@available(*, unavailable)
-extension __ContiguousArrayStorageBase: Sendable {}