collection-librarian should attempt to remove unneeded tracks, releases, albums, and artists

[Chris Wagner]

E.g., if a given artist has no associated albums and no associated tracks, then we should attempt to remove him from the database.

For the case of tracks -- at least, and perhaps for some other types -- we'll need to consider that the given track may have an association in the "track_suggestions" table. We ought to just catch and log any database errors that occur when attempting to remove such a track. In fact, we should catch all database errors when trying to remove any object.

[Chris Wagner (chris-wagner)]

It looks like unassociated artists were already being removed, actually; however, we weren't removing unassociated albums, so it's unlikely any artist would be removed in this way.

[Chris Wagner (chris-wagner)]

Addressed in revision 835.