Remove useless MERGE test

This was trying to exercise an ERROR we don't actually have.

Backpatch to 15.

Reported by Teja Mupparti <Tejeswar.Mupparti@microsoft.com>
Discussion: https://postgr.es/m/SN6PR2101MB1040BDAF740EA4389484E92BF0079@SN6PR2101MB1040.namprd21.prod.outlook.com

src/test/regress/expected/merge.out

@@ -843,22 +843,6 @@ SELECT * FROM wq_target;
    1 |     499
 (1 row)
 
--- test preventing WHEN conditions from writing to the database
-create or replace function merge_when_and_write() returns boolean
-language plpgsql as
-$$
-BEGIN
-	INSERT INTO target VALUES (100, 100);
-	RETURN TRUE;
-END;
-$$;
-BEGIN;
-MERGE INTO wq_target t
-USING wq_source s ON t.tid = s.sid
-WHEN MATCHED AND (merge_when_and_write()) THEN
-	UPDATE SET balance = t.balance + s.balance;
-ROLLBACK;
-drop function merge_when_and_write();
 DROP TABLE wq_target, wq_source;
 -- test triggers
 create or replace function merge_trigfunc () returns trigger

src/test/regress/sql/merge.sql

@@ -564,24 +564,6 @@ WHEN MATCHED AND t.tableoid >= 0 THEN
 	UPDATE SET balance = t.balance + s.balance;
 SELECT * FROM wq_target;
 
--- test preventing WHEN conditions from writing to the database
-create or replace function merge_when_and_write() returns boolean
-language plpgsql as
-$$
-BEGIN
-	INSERT INTO target VALUES (100, 100);
-	RETURN TRUE;
-END;
-$$;
-
-BEGIN;
-MERGE INTO wq_target t
-USING wq_source s ON t.tid = s.sid
-WHEN MATCHED AND (merge_when_and_write()) THEN
-	UPDATE SET balance = t.balance + s.balance;
-ROLLBACK;
-drop function merge_when_and_write();
-
 DROP TABLE wq_target, wq_source;
 
 -- test triggers