Drop support for treating MusicBrainz ids as track unique ids

No comments on my mail inquiry, which I treat as a consent. Since 2.6
the SqlScanResultProcessor should cope with changed track ids just fine.

CHANGES:
 * Problematic support for treating MusicBrainz ids as track unique ids
   was dropped; should avoid surprising "Duplicate Tracks Found" errors.

BUG: 315329