Remove art when removing duplicate album

When importing a higher quality version of an album already in my library, beet offered to remove the old version, which is very handy. But it left behind the old album's directory with the cover file in it. I would have expected beet to remove it completely.

Start by reproducing the duplicate-album replacement flow described in the issue and trace the code that removes the old album. Confirm the expected result by checking that the old album directory, including its cover file, is removed completely.

Written by the indexing model from the issue text.