Prefer std::condition_t vs preprocessor in AsyncSocket

Summary: [Folly] Prefer `std::condition_t` vs preprocessor in `AsyncSocket`.

Reviewed By: bschlinker, lnicco

Differential Revision: D22975614

fbshipit-source-id: 562d1b794ec3dbab538ad507b2ec5d0fe07c7757

folly/io/async/AsyncSocket.h

@@ -1398,13 +1398,10 @@ class AsyncSocket : public AsyncTransport {
   //
   // Use small_vector to avoid heap allocation for up to two observers, unless
   // mobile, in which case we fallback to std::vector to prioritize code size.
-#if !FOLLY_MOBILE
-  using LifecycleObserverVecImpl =
-      folly::small_vector<AsyncTransport::LifecycleObserver*, 2>;
-#else
-  using LifecycleObserverVecImpl =
-      std::vector<AsyncTransport::LifecycleObserver*>;
-#endif
+  using LifecycleObserverVecImpl = conditional_t<
+      !kIsMobile,
+      folly::small_vector<AsyncTransport::LifecycleObserver*, 2>,
+      std::vector<AsyncTransport::LifecycleObserver*>>;
   LifecycleObserverVecImpl lifecycleObservers_;
 
   // Pre-received data, to be returned to read callback before any data from the