bluetooth: Remove setup plugins support

It was used by a test plugin we ship ourselves for Geoclue 1.x,
and NetworkManager. Given that NetworkManager doesn't make use
of it anymore, let's remove it.
sharing: Use g-s-d's rfkill for Bluetooth

https://bugzilla.gnome.org/show_bug.cgi?id=719500
power: Use g-s-d's rfkill for Bluetooth

https://bugzilla.gnome.org/show_bug.cgi?id=691151
shell: Assert when the panel is empty

Rather than getting a segfault.

https://bugzilla.gnome.org/show_bug.cgi?id=719711
bluetooth: Updated design

With the majority of the implementation living in gnome-bluetooth.

https://bugzilla.gnome.org/show_bug.cgi?id=719565