Cycles: no need in spin lock in memory statistics

This functions are called from device code which is guaranteed
not to be called simultaneously from different threads.

intern/cycles/util/util_thread.h

@@ -33,9 +33,6 @@ typedef boost::mutex thread_mutex;
 typedef boost::mutex::scoped_lock thread_scoped_lock;
 typedef boost::condition_variable thread_condition_variable;
 
-/* use boost for spinlocks as well */
-typedef boost::detail::spinlock spin_lock;
-
 /* own pthread based implementation, to avoid boost version conflicts with
  * dynamically loaded blender plugins */