Remove configuration preprocessor symbols 'THREAD'

and 'THREAD_SAFE_CLIENT'.
  
As of MySQL 5.5, we no longer support non-threaded
builds.   This patch removes all references to the
obsolete THREAD and THREAD_SAFE_CLIENT preprocessor
symbols.  These were used to distinguish between
threaded and non-threaded builds.

include/my_bitmap.h

@@ -33,9 +33,7 @@ typedef struct st_bitmap
      thread_safe flag in bitmap_init was set.  Otherwise, we optimize by not
      acquiring the mutex
    */
-#ifdef THREAD
   mysql_mutex_t *mutex;
-#endif
 } MY_BITMAP;
 
 #ifdef	__cplusplus