This was a vestige from when we used to need the unittest2 library for pre-2.7
compatibility. Now that we require Python 2.7, we aren't using that library
and this indirection wasn't doing any good.
* Call AutotagStub.restore() during the tearDown of
test_mbsubmit.MBSubmitPluginTest and test_plugins.PromptChoicesTest,
which could potentially lead to other tests calling mocked versions
of autotag.mb.match_album, .match_track, .album_for_id and .track_for_id
instead of the real functions.
