No need to optimize MicroSpinLock::lock in folly/futures/

Summary: [Folly] No need to optimize `MicroSpinLock::lock` in `folly/futures/` after {D7828158} (facebook/folly@6f9d8a145e1c37b2b47ff91491c647fe321de787).

Reviewed By: nbronson

Differential Revision: D7843192

fbshipit-source-id: b40bcc0abcfed561a141864e135d72fe33f4067d

folly/futures/detail/Core.h

@@ -64,11 +64,7 @@ enum class State : uint8_t {
 struct SpinLock : private MicroSpinLock {
   SpinLock() : MicroSpinLock{0} {}
 
-  void lock() {
-    if (!MicroSpinLock::try_lock()) {
-      MicroSpinLock::lock();
-    }
-  }
+  using MicroSpinLock::lock;
   using MicroSpinLock::unlock;
 };
 static_assert(sizeof(SpinLock) == 1, "missized");