deskutils/xdg-desktop-portal: disable (broken) support for flatpak and snaps

Make it possible to run firefox with KDE filedialogs.

The call for parse_app_info_from_flatpak_info uses the /proc/$PID/root
file to access the flatpak information file.

We neither have support for /proc/$PID/root nor flatpaks or snaps.

With this fix and deskutils/plasma5-xdg-desktop-portal-kde it is now possible
to run firefox using the KDE file dialogs.

GTK_USE_PORTAL=1 firefox