[MikeKL]

Can texturecache remove all the artwork that is no longer has entries in the database?

Yes, and from following specific extract, from 1st post in this thread :-

[p, P] Prune texture cache by identifying (p) or removing (P) accumulated cruft such as image previews, previously deleted movies/tv shows/music whose artwork remains in the texture cache even after cleaning the database. Essentially, remove any cached file that is no longer associated with an entry in the media library and is therefore just wasting disk space.

So assuming you have texturecache installed, ssh into your Rpi from computer and then run following command.
(My basic understanding of simplest way to do what you want)

Code:

./texturecache.py P