Revert D6725091: [Folly] Use thread-local in RequestContext::getStaticContext

Summary:
This reverts commit 9979f39677284b1051cb109b461097495d77ca17

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Differential Revision: D6725091

fbshipit-source-id: f1e3c80c869aa47684f5fbe79528e68174dee568

folly/io/async/Request.cpp

@@ -141,16 +141,10 @@ std::shared_ptr<RequestContext> RequestContext::setContext(
 }
 
 std::shared_ptr<RequestContext>& RequestContext::getStaticContext() {
-  using T = std::shared_ptr<RequestContext>;
-#ifdef FOLLY_TLS
-  alignas(alignof(T)) static FOLLY_TLS unsigned char storage[sizeof(T)];
-  static FOLLY_TLS T* singleton;
-  return singleton ? *singleton : *(singleton = new (storage) T());
-#else
-  struct PrivateTag {};
-  static SingletonThreadLocal<T, PrivateTag> singleton;
+  using SingletonT = SingletonThreadLocal<std::shared_ptr<RequestContext>>;
+  static SingletonT singleton;
+
   return singleton.get();
-#endif
 }
 
 RequestContext* RequestContext::get() {