Don't superfluously materialize slot after DELETE from an FDW.

Previously that was needed to safely store the table oid, but after
b8d71745 that's not necessary anymore.

Author: Andres Freund

src/backend/executor/nodeModifyTable.c

@@ -655,7 +655,6 @@ ExecDelete(ModifyTableState *mtstate,
 		 */
 		if (TTS_EMPTY(slot))
 			ExecStoreAllNullTuple(slot);
-		ExecMaterializeSlot(slot);
 
 		slot->tts_tableOid = RelationGetRelid(resultRelationDesc);
 	}