Remove artists and genres from the database when the last track associated with them is removed
Reason for the change
Prevents the Artists and Genres views from displaying artists/genres with no tracks
Test plan
Updated the unit tests
To test manually:
- Open the artists view
- Drag and drop a track with a new artist into one of the indexed folders
- Ensure the artist is added to the view
- Remove the track
- Ensure the artist has been removed from the view