Revert "[SILGen] Implement NSString -> String bridging through _ObjectiveCBridgeable."

This reverts commit aa9cc23743.

stdlib/public/core/BridgeObjectiveC.swift

@@ -73,40 +73,6 @@ public protocol _ObjectiveCBridgeable {
     source: _ObjectiveCType,
     result: inout Self?
   ) -> Bool
-
-  /// Bridge from an Objective-C object of the bridged class type to a
-  /// value of the Self type.
-  ///
-  /// This bridging operation is used for unconditional bridging when
-  /// interoperating with Objective-C code, either in the body of an
-  /// Objective-C thunk or when calling Objective-C code, and may
-  /// defer complete checking until later. For example, when bridging
-  /// from `NSArray` to `Array<Element>`, we can defer the checking
-  /// for the individual elements of the array.
-  ///
-  /// \param source The Objective-C object from which we are
-  /// bridging. This optional value will only be `nil` in cases where
-  /// an Objective-C method has returned a `nil` despite being marked
-  /// as `_Nonnull`/`nonnull`. In most such cases, bridging will
-  /// generally force the value immediately. However, this gives
-  /// bridging the flexibility to substitute a default value to cope
-  /// with historical decisions, e.g., an existing Objective-C method
-  /// that returns `nil` to for "empty result" rather than (say) an
-  /// empty array. In such cases, when `nil` does occur, the
-  /// implementation of `Swift.Array`'s conformance to
-  /// `_ObjectiveCBridgeable` will produce an empty array rather than
-  /// dynamically failing.
-  static func _unconditionallyBridgeFromObjectiveC(source: _ObjectiveCType?)
-      -> Self
-}
-
-public extension _ObjectiveCBridgeable {
-  static func _unconditionallyBridgeFromObjectiveC(source: _ObjectiveCType?)
-      -> Self {
-    var result: Self? = nil
-    _forceBridgeFromObjectiveC(source!, result: &result)
-    return result!
-  }
 }
 
 //===--- Bridging for metatypes -------------------------------------------===//