stdlib: Address a couple of ABI FIXMEs by deleting code

We have an implicit conversion to AnyHashable, so there's no
need to have the special subscript on Dictionary at all.

stdlib/public/core/HashedCollectionsAnyHashableExtensions.swift

@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+//===----------------------------------------------------------------------===//
+// Convenience APIs for Set<AnyHashable>
+//===----------------------------------------------------------------------===//
+
+extension Set where Element == AnyHashable {
+  public mutating func insert<ConcreteElement : Hashable>(
+    _ newMember: ConcreteElement
+  ) -> (inserted: Bool, memberAfterInsert: ConcreteElement) {
+    let (inserted, memberAfterInsert) =
+      insert(AnyHashable(newMember))
+    return (
+      inserted: inserted,
+      memberAfterInsert: memberAfterInsert.base as! ConcreteElement)
+  }
+
+  @discardableResult
+  public mutating func update<ConcreteElement : Hashable>(
+    with newMember: ConcreteElement
+  ) -> ConcreteElement? {
+    return update(with: AnyHashable(newMember))
+      .map { $0.base as! ConcreteElement }
+  }
+
+  @discardableResult
+  public mutating func remove<ConcreteElement : Hashable>(
+    _ member: ConcreteElement
+  ) -> ConcreteElement? {
+    return remove(AnyHashable(member))
+      .map { $0.base as! ConcreteElement }
+  }
+}