Remove any-user DML capability from allow_system_table_mods

Previously, allow_system_table_mods allowed a non-superuser to do DML
on a system table without further permission checks.  This has been
removed, as it was quite inconsistent with the rest of the meaning of
this setting.  (Since allow_system_table_mods was previously only
accessible with a server restart, it is unlikely that anyone was using
this possibility.)
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/8b00ea5e-28a7-88ba-e848-21528b632354%402ndquadrant.com

src/backend/catalog/aclchk.c

@@ -3851,7 +3851,7 @@ pg_class_aclmask(Oid table_oid, Oid roleid,
 
 	/*
 	 * Deny anyone permission to update a system catalog unless
-	 * pg_authid.rolsuper is set.  Also allow it if allowSystemTableMods.
+	 * pg_authid.rolsuper is set.
 	 *
 	 * As of 7.4 we have some updatable system views; those shouldn't be
 	 * protected in this way.  Assume the view rules can take care of
@@ -3860,8 +3860,7 @@ pg_class_aclmask(Oid table_oid, Oid roleid,
 	if ((mask & (ACL_INSERT | ACL_UPDATE | ACL_DELETE | ACL_TRUNCATE | ACL_USAGE)) &&
 		IsSystemClass(table_oid, classForm) &&
 		classForm->relkind != RELKIND_VIEW &&
-		!superuser_arg(roleid) &&
-		!allowSystemTableMods)
+		!superuser_arg(roleid))
 	{
 #ifdef ACLDEBUG
 		elog(DEBUG2, "permission denied for system catalog update");