core: Refactor or remove existing `mb_albumid` check from core

I just noticed we still have a bit of musicbrainz specific logic in the core. Seems a bit inconsistent to me and there might be a better way to resolve this 🤔

https://github.com/beetbox/beets/blob/08081f3c440c3aff2ff66931af645c92edfceb47/beets/autotag/match.py#L422-L442

This needs a bit of a rework in my opinion. I think metadata plugins should handle this on their own or we need to properly define a public interface for registering `_id` fields.

@snejus For visibility.

Start with beets/autotag/match.py at lines 422-442 and trace how the existing mb_albumid check is used. Compare the current metadata-plugin behavior and consider the two directions named in the issue: plugin-owned handling or a public interface for _id fields. Done means the core no longer contains inconsistent MusicBrainz-specific handling under an agreed design.

Written by the indexing model from the issue text.