list-remotes: Default to list only system remotes

This matches what install-* would use and also makes the bash completion
more sane.

app/xdg-app-builtins-list-remotes.c

@@ -59,7 +59,6 @@ xdg_app_builtin_list_remotes (int argc, char **argv, GCancellable *cancellable,
 
   if (!opt_user && !opt_system)
     {
-      opt_user = TRUE;
       opt_system = TRUE;
     }