Remove the DCB write queue locking

Since only one thread can append to the DCBs write queue at a time, there
is no need to lock it.

server/modules/protocol/MySQL/MySQLBackend/mysql_backend.c

@@ -884,13 +884,11 @@ static int gw_write_backend_event(DCB *dcb)
         uint8_t* data = NULL;
         bool com_quit = false;
 
-        spinlock_acquire(&dcb->writeqlock);
         if (dcb->writeq)
         {
             data = (uint8_t *) GWBUF_DATA(dcb->writeq);
             com_quit = MYSQL_IS_COM_QUIT(data);
         }
-        spinlock_release(&dcb->writeqlock);
 
         if (data)
         {