This reverts commit c9e02bfd8a.
disable notifications at this level did not cause fractal (gtk app) to
send its notifications to swaync. instead, it still tried to deliver to
the Portal, where the Portal wasn't expecting anything and just returned
an error to fractal.
setting `GNOTIFICATION_BACKEND = "freedesktop"` seems to be the correct
way to get gtk apps to behave as desired with their notifications.
