Fix for bug #5890 "Triggers fail for DELETE without WHERE".

If we have DELETE with always true WHERE clause we should not use 
optimized delete_all_rows() method for tables with DELETE triggers,
because in this case we will lose side-effect of deletion.

sql/sql_trigger.h

@@ -58,5 +58,11 @@ public:
   static bool check_n_load(THD *thd, const char *db, const char *table_name,
                            TABLE *table);
 
+  bool has_delete_triggers()
+  {
+    return (bodies[TRG_EVENT_DELETE][TRG_ACTION_BEFORE] ||
+            bodies[TRG_EVENT_DELETE][TRG_ACTION_AFTER]);
+  }
+
   friend class Item_trigger_field;
 };