At the time I searched and searched and searched for a solution, and then I gave up. All I could find was the usual utter bollocks from people who have no idea what they're talking about. Warnings about how removing the music database will leave the whole application in an unusable state. What a load of crap. I gave up because I couldn't be arsed with this chew. Well, it's annoying me again.
Logically: a new user running Rhythmbox doesn't have a music database; it has to be created. Therefore I should be able to remove the database and a new one will be created.
$ mv ~/.local/share/rhythmbox/rhythmdb.xml ~/.local/share/rhythmbox/rhythmdb.xml.old
Predictably, the new database is half the size of the old one.