Settings: remove AccountsService fallback

[lenemter]

It doesn't make sense to keep this, we use settings portal nowadays

[tintou]

Note that non-flatpak apps don't use portals

[lenemter]

@tintou That's not true. Even non-sandboxed apps use Portals, that's the reason why if you launch granite apps without portals, you get 30 seconds startup delay. I just ported this patch to 7.6.0 to build it on my OS 8 and I can confirm that.

[danirabbit]

Since this is a break I think it needs to come with an 8.0.0 version bump

[Marukesu]

We still need a non-portal source of truth for the portals themselves. We shouldn't be removing the accounts service path without a replacement for the code that executes before portals are available.

[jeremypw]

Not sure why but Terminal headless unit tests fail on unstable build in Gtk4 with the message:

# DEBUG: Settings.vala:172: cannot use the portal, using the AccountsService: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.portal.Desktop was not provided by any .service files
not ok /application/action/new-window - FATAL-CRITICAL: Settings.vala:114: Could not connect: No such file or directory

So the AccountsService seems to be needed in that case, unless someone can advise how to get it to use the portal.