bluetooth: Remove unused 'had-bluetooth-devices-setup' setting

bluetooth: Remove unused 'had-bluetooth-devices-setup' setting

Now that we always show the Bluetooth switch when there's a Bluetooth
adapter hiding somewhere, remove all the settings it used as storage.

If the user wants to really really disable any Bluetooth adapters in
their system, the gnome-bluetooth documentation explains how to do that
irrespective of GNOME:
https://gitlab.gnome.org/GNOME/gnome-bluetooth/#multiple-bluetooth-adapters

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2488>