Attempt to address the comments from sampsyo

Undoes the changes to autotag/hooks.py

If trackdisambig is not set to None at some point in time, the
test case for a non-existent trackdisambig fails. as such, try
to get the disambiguation with a default of None when setting
trackdisambig. Not sure if this is reasonable.

beets/autotag/hooks.py

@@ -163,7 +163,7 @@ class TrackInfo(AttrDict):
                  composer=None, composer_sort=None, arranger=None,
                  track_alt=None, work=None, mb_workid=None,
                  work_disambig=None, bpm=None, initial_key=None, genre=None,
-                 trackdisambig=None, **kwargs):
+                 **kwargs):
         self.title = title
         self.track_id = track_id
         self.release_track_id = release_track_id
@@ -191,7 +191,6 @@ class TrackInfo(AttrDict):
         self.bpm = bpm
         self.initial_key = initial_key
         self.genre = genre
-        self.trackdisambig = trackdisambig
         self.update(kwargs)
 
     # As above, work around a bug in python-musicbrainz-ngs.

beets/autotag/mb.py

@@ -223,8 +223,7 @@ def track_info(recording, index=None, medium=None, medium_index=None,
     if recording.get('length'):
         info.length = int(recording['length']) / (1000.0)
 
-    if recording.get('disambiguation'):
-        info.trackdisambig = recording.get('disambiguation')
+    info.trackdisambig = recording.get('disambiguation', None)
 
     lyricist = []
     composer = []