Use C++ manner for unused param instead of C manner

sql/sql_class.cc

@@ -301,9 +301,8 @@ void thd_set_thread_stack(THD *thd, char *stack_start)
 
   @param thd                       THD object
 */
-void thd_lock_connection_data(THD *thd)
+void thd_lock_connection_data(THD *)
 {
-  (void)thd;
   mysql_mutex_lock(&LOCK_thread_count);
 }
 
@@ -313,9 +312,8 @@ void thd_lock_connection_data(THD *thd)
 
   @param thd                       THD object
 */
-void thd_unlock_connection_data(THD *thd)
+void thd_unlock_connection_data(THD *)
 {
-  (void)thd;
   mysql_cond_broadcast(&COND_thread_count);
   mysql_mutex_unlock(&LOCK_thread_count);
 }