don't penalize Discogs matches for lacking MBIDs
don't penalize Discogs matches for lacking MBIDs

This removes the distance component for MBIDs for matches that don't have
MBIDs. (Previously, when importing an album that already has MBIDs, Discogs
would be unfairly penalized.)

google-code-export/beets,google-code-export/beets,google-code-export/beets