Remove the SyncPoint usage in the destructor of PosixEnv

Summary:
Remove the SyncPoint usage in the destructor of PosixEnv as none
of any active tests is using it.

SyncPoint is a test-only utility class, and it's a static varible.
As a result, using SyncPoint in the destructor of PosixEnv will
make default Env depends on SyncPoint.  Removing such dependency
could solve the problem crash issue only reproducable in Mac
environment.

Test Plan: OPT=-DTRAVIS V=1 make -j4 check on Mac environment

Reviewers: sdong, anthony

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54333

util/env_posix.cc

@@ -131,7 +131,6 @@ class PosixEnv : public Env {
     // All threads must be joined before the deletion of
     // thread_status_updater_.
     delete thread_status_updater_;
-    TEST_SYNC_POINT("PosixEnv::~PosixEnv():End");
   }
 
   void SetFD_CLOEXEC(int fd, const EnvOptions* options) {