remove port hack using ThreadLocal in fibers only on Apple
remove port hack using ThreadLocal in fibers only on Apple

Summary: The portability hack was landed 7 years ago in {D2151318 (https://github.com/facebook/folly/commit/2bf6f465d9d9b418c09f02a06842a34a67123907)}. Presumably all supported Apple targets now have `thread_local`.

Differential Revision: D37116931

fbshipit-source-id: b5d55e576be1a0fddc0c91193f098fcd15833035

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly