pgsql: Remove useless MERGE test

Remove useless MERGE test This was trying to exercise an ERROR we don't actually have. Backpatch to 15. Reported by Teja Mupparti Discussion: https://postgr.es/m/sn6pr2101mb1040bdaf740ea4389484e92bf0...@sn6pr2101mb1040.namprd21.prod.outlook.com Branch -- REL_15_STABLE Details --- htt

pgsql: Remove useless MERGE test

Remove useless MERGE test This was trying to exercise an ERROR we don't actually have. Backpatch to 15. Reported by Teja Mupparti Discussion: https://postgr.es/m/sn6pr2101mb1040bdaf740ea4389484e92bf0...@sn6pr2101mb1040.namprd21.prod.outlook.com Branch -- master Details --- https://gi