Remove last use of @_unsafeSendable in the concurrency library

Author: DougGregor

stdlib/public/Concurrency/Task.swift

@@ -760,7 +760,7 @@ internal func _asyncMainDrainQueue() -> Never
 internal func _getMainExecutor() -> Builtin.Executor
 
 @available(SwiftStdlib 5.1, *)
-public func _runAsyncMain(@_unsafeSendable _ asyncFun: @escaping () async throws -> ()) {
+public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
   Task.detached {
     do {
 #if !os(Windows)