Remove Discogs label disambiguation number

[cklamra]

At the moment Discogs disambiguation number for the artist tag is removed on import, as suggested here and implemented here. A disambiguation number may also be present in the label tag (example, Discogs database guidelines (see 4.3.1)).
Perhaps the disambiguation number should also be removed from the label entry?

[henry-oberholtzer]

It appears that the artist disambiguation removal currently happens in beets/metadata_plugins.py:MetadataSourcePlugin, a different location from where it was in #289.

This seems unusual to me since the number disambiguation is a pretty discogs specific quirk, and doesn't appear on any of the other metadata sources like Deezer, Spotify, BeatPort, etc. It might be worth moving that regex down to the child class.

I think adding the regex remove at the end of DiscogsPlugin:get_album_info could be good - maybe adding a default config option for "preserve_discogs_disambiguation: yes" for those who would like it to do so.

Happy to make a PR for this to fix it and bring more consistency and configurability to the Discogs plugin behavior.