BookStats remaining after books have been deleted

[cblanken]

Description I'm not sure if it's intended but it seems like BookStat records remain even when a Book has been deleted.

To Reproduce

Steps to reproduce the behavior, e.g.:

1. Start Lute with default database
2. Delete books
3. Check bookstats in SQLite Browser or via sqlite
4. See bookstats for deleted books still exist

Screenshots

For example, the bookstats after having deleted all but a few of the default books.