temp/gtk+3.0: drop

Since we now install papers by default instead of Evince, every single app we install from GNOME packages is either adaptive or GTK4.

The only real difference I have seen is that portals with Phosh are still using GTK3. Given that Phosh allows to fit windows to the view, I would consider this an acceptable regression, as there is a simple and common workaround available.

Fixes #1479 (closed)

Depends !5234 (merged)