main/postmarketos-ui-phosh: clean up dconf overrides
ffea64b8 missed to remove some dconf configuration.
Fix warning:
(flatpak install:4864): dconf-WARNING **: 21:54:52.405: unable to open file '/etc/dconf/db/postmarketos': L’ouverture du fichier « /etc/dconf/db/postmarketos » a échoué : échec de open() : No such file or directory; expect degraded performance