sql: remove redundant pointer in TriggerStep

The trigger pointer in the sql_trigger structure is a dead code
now, so it could be deleted.

Needed for #4343

src/box/sql/fk_constraint.c

@@ -895,7 +895,6 @@ fk_constraint_action_trigger(struct Parse *pParse, struct space_def *def,
 		step->op = TK_UPDATE;
 	}
 
-	step->trigger = trigger;
 	if (is_update) {
 		fk->on_update_trigger = trigger;
 		trigger->op = TK_UPDATE;

src/box/sql/sqlInt.h

@@ -2366,8 +2366,6 @@ struct sql_trigger {
 struct TriggerStep {
 	u8 op;			/* One of TK_DELETE, TK_UPDATE, TK_INSERT, TK_SELECT */
 	u8 orconf;		/* ON_CONFLICT_ACTION_ROLLBACK etc. */
-	/** The trigger that this step is a part of */
-	struct sql_trigger *trigger;
 	Select *pSelect;	/* SELECT statement or RHS of INSERT INTO SELECT ... */
 	char *zTarget;		/* Target table for DELETE, UPDATE, INSERT */
 	Expr *pWhere;		/* The WHERE clause for DELETE or UPDATE steps */

src/box/sql/trigger.c

@@ -170,7 +170,6 @@ sql_trigger_finish(struct Parse *parse, struct TriggerStep *step_list,
 	char *trigger_name = trigger->zName;
 	trigger->step_list = step_list;
 	while (step_list != NULL) {
-		step_list->trigger = trigger;
 		step_list = step_list->pNext;
 	}