Use flexible attribute for track_alt

No need for a built-in field for a simple string-type optional field like
this.

beets/library.py

@@ -423,7 +423,6 @@ class Item(LibModel):
         'month':                types.PaddedInt(2),
         'day':                  types.PaddedInt(2),
         'track':                types.PaddedInt(2),
-        'track_alt':            types.STRING,
         'tracktotal':           types.PaddedInt(2),
         'disc':                 types.PaddedInt(2),
         'disctotal':            types.PaddedInt(2),

docs/reference/pathformat.rst

@@ -197,7 +197,6 @@ Ordinary metadata:
 * original_year, original_month, original_day: The release date of the original
   version of the album.
 * track
-* track_alt
 * tracktotal
 * disc
 * disctotal

test/test_edit.py

@@ -276,7 +276,7 @@ class EditDuringImporterTest(TerminalImportSessionSetup, unittest.TestCase,
                              ImportHelper, TestHelper, EditMixin):
     """TODO
     """
-    IGNORED = ['added', 'album_id', 'id', 'mtime', 'path', 'track_alt']
+    IGNORED = ['added', 'album_id', 'id', 'mtime', 'path']
 
     def setUp(self):
         self.setup_beets()

test/test_mb.py

@@ -189,6 +189,7 @@ class MBAlbumInfoTest(_common.TestCase):
             'position': '2',
             'track-list': second_track_list,
         })
+
         d = mb.album_info(release)
         self.assertEqual(d.mediums, 2)
         t = d.tracks