Remove explicit unavailable sendable from keypath

Author: Azoy

stdlib/public/core/KeyPath.swift

@@ -262,16 +262,10 @@ public class AnyKeyPath: Hashable, _AppendKeyPath {
   }
 }
 
-@available(*, unavailable)
-extension AnyKeyPath: Sendable {}
-
 /// A partially type-erased key path, from a concrete root type to any
 /// resulting value type.
 public class PartialKeyPath<Root>: AnyKeyPath { }
 
-@available(*, unavailable)
-extension PartialKeyPath: Sendable {}
-
 // MARK: Concrete implementations
 internal enum KeyPathKind { case readOnly, value, reference }
 
@@ -377,9 +371,6 @@ public class KeyPath<Root, Value>: PartialKeyPath<Root> {
   }
 }
 
-@available(*, unavailable)
-extension KeyPath: Sendable {}
-
 /// A key path that supports reading from and writing to the resulting value.
 public class WritableKeyPath<Root, Value>: KeyPath<Root, Value> {
   // MARK: Implementation detail
@@ -448,9 +439,6 @@ public class WritableKeyPath<Root, Value>: KeyPath<Root, Value> {
   }
 }
 
-@available(*, unavailable)
-extension WritableKeyPath: Sendable {}
-
 /// A key path that supports reading from and writing to the resulting value
 /// with reference semantics.
 public class ReferenceWritableKeyPath<
@@ -522,9 +510,6 @@ public class ReferenceWritableKeyPath<
   }
 }
 
-@available(*, unavailable)
-extension ReferenceWritableKeyPath: Sendable {}
-
 // MARK: Implementation details
 
 internal enum KeyPathComponentKind {