With the porting of gnome-bluetooth to the new GListModel API the
behavior regarding removing adapters changed: It now no longer
guarantees to emit "device-removed" signals for the paired devices when
the adapter gets removed.
This means we need to do that ourselves now, so clear the list of
connected signals when the default adapter changes.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2214>