backend: Remove old backends API

As far as I know, Mopidy-VKontakte is the only backend extension that hasn't
been updated to use the new API yet. They have a PR ready for merging that does
the update for them.
ext: Remove old extension API

As far as I know, all extensions except Mopidy-Arcam (not currently
maintained) and Mopidy-VKontakte already use the new API. Both of the
remaining extensions got open pull requests with the needed changes to use the
new API.