Revert "Implement AsyncIteratorProtocol.next() in terms of next(isolation:)."

This reverts commit bf38117.

Author: DougGregor

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -111,9 +111,8 @@ public protocol AsyncIteratorProtocol<Element, Failure> {
 
 @available(SwiftStdlib 5.1, *)
 extension AsyncIteratorProtocol {
-  /// Default implementation of `next(isolation:)` in terms of `next()`, which
-  /// is required to maintain backward compatibility with existing async
-  /// iterators.
+  /// Default implementation of `next()` in terms of `next()`, which is
+  /// required to maintain backward compatibility with existing async iterators.
   @available(SwiftStdlib 6.0, *)
   @inlinable
   public mutating func next(isolation actor: isolated (any Actor)?) async throws(Failure) -> Element? {
@@ -124,19 +123,3 @@ extension AsyncIteratorProtocol {
     }
   }
 }
-
-@available(SwiftStdlib 5.1, *)
-extension AsyncIteratorProtocol {
-  /// Default implementation of `next()` in terms of `next(isolation:)`, which
-  /// is required to maintain backward compatibility with existing async
-  /// iterators.
-  @available(SwiftStdlib 6.0, *)
-  @inlinable
-  public mutating func next() async throws(Failure) -> Element? {
-#if $OptionalIsolatedParameters
-    return try await next(isolation: nil)
-#else
-    fatalError("unsupported compiler")
-#endif
-  }
-}

test/Concurrency/async_sequence_rethrows.swift

@@ -75,13 +75,3 @@ enum MyError: Error {
 func getASequence() -> any AsyncSequence<Data, MyError> {
     return ErrorSequence<Data, _>(throwError: MyError.foo) // ERROR: Cannot convert return expression of type 'any Error' to return type 'MyError'
 }
-
-// Test the default implementation of next() in terms of next(isolation:).
-struct AsyncIteratorWithOnlyNextIsolation: AsyncIteratorProtocol {
-  public mutating func next(isolation: (any Actor)?) throws(MyError) -> Int? { 0 }
-}
-
-// Test the default implementation of next(isolation:) in terms of next().
-struct AsyncIteratorWithOnlyNext: AsyncIteratorProtocol {
-  public mutating func next() throws(MyError) -> Int? { 0 }
-}