kill SpinLock

It is unused (and not implemented on OSX) so killing it seems good. It was introduced in rFBCODEe991769c74e876f345e2cd92edf1d970801e884b with no users.

hphp/util/mutex.h

@@ -206,42 +206,6 @@ public:
 
 ///////////////////////////////////////////////////////////////////////////////
 
-class SpinLock {
-#ifdef DEBUG
-  Rank m_rank;
-#endif
-public:
-  explicit SpinLock(Rank rank = RankUnranked)
-#ifdef DEBUG
-    : m_rank(rank)
-#endif
-  {
-    pthread_spin_init(&m_spinlock, 0);
-  }
-  ~SpinLock() {
-    pthread_spin_destroy(&m_spinlock);
-  }
-
-  void lock() {
-    pushRank(m_rank);
-    pthread_spin_lock(&m_spinlock);
-  }
-  void unlock() {
-    popRank(m_rank);
-    pthread_spin_unlock(&m_spinlock);
-  }
-
-  pthread_spinlock_t &getRaw() { return m_spinlock;}
-
-private:
-  SpinLock(const SpinLock &); // suppress
-  SpinLock &operator=(const SpinLock &); // suppress
-
-  pthread_spinlock_t m_spinlock;
-};
-
-///////////////////////////////////////////////////////////////////////////////
-
 /**
  * Read-write lock wrapper.
  */