spotify: Fix GStreamer warning on seek (fixes #227)

Don't call audio.{prepare_change,start_playback}() before/after seek. This does
not seem to have any effect on functionality, and avoids Gstreamer failing to
disconnect the "notify::source" signal handler, which again causes a warning.

mopidy/backends/spotify/playback.py

@@ -52,12 +52,8 @@ class SpotifyPlaybackProvider(base.BasePlaybackProvider):
         return self.seek(time_position)
 
     def seek(self, time_position):
-        self.audio.prepare_change()
         self.backend.spotify.session.seek(time_position)
-        self.audio.start_playback()
-
         self._timer.seek(time_position)
-
         return True
 
     def stop(self):