screenshot: Remove GNOME Shell implementation

GNOME Shell has started restricting access for it's Screenshot D-Bus API
to internal components only [1] for security reasons. In other words,
this will start failing, so remove it in favor of just using the
freedesktop portal, which should always work.

[1]: gnome-shell!1970
8 jobs for !527 with nielsdg/remove-gnome-shell in 34 minutes and 26 seconds (queued for 3 seconds)
merge request