Remove 'msg' parameter from convert_tuples_by_name

The message was included as a parameter when this function was added in
dcb2bda9b704, but I don't think it has ever served any useful purpose.
Let's stop spreading it pointlessly.

Reviewed by Amit Langote and Peter Eisentraut.

Discussion: https://postgr.es/m/20190806224728.GA17233@alvherre.pgsql

src/backend/executor/nodeModifyTable.c

@@ -1959,8 +1959,7 @@ ExecSetupChildParentMapForSubplan(ModifyTableState *mtstate)
 	{
 		mtstate->mt_per_subplan_tupconv_maps[i] =
 			convert_tuples_by_name(RelationGetDescr(resultRelInfos[i].ri_RelationDesc),
-								   outdesc,
-								   gettext_noop("could not convert row type"));
+								   outdesc);
 	}
 }