simplify usingJEMalloc() when FOLLY_SANITIZE is true

Summary: Sanitizers always intercept the allocator, so there's no reason to do the elaborate `usingJEMalloc()` logic -- it will always fail.

Reviewed By: yfeldblum

Differential Revision: D20056781

fbshipit-source-id: c0d4a3b103e26bacc0a6589f8d54b4a2ec85f771

folly/memory/Malloc.h

@@ -67,7 +67,7 @@ namespace folly {
 /**
  * Determine if we are using jemalloc or not.
  */
-#if defined(FOLLY_ASSUME_NO_JEMALLOC)
+#if defined(FOLLY_ASSUME_NO_JEMALLOC) || FOLLY_SANITIZE
   inline bool usingJEMalloc() noexcept {
     return false;
   }