[videodb] clear out art from tvshow/seasons that were combined

In the v85 database update we combined seasons/shows from multipaths, but this would have left links in the art table, as the triggers would not have kicked off due to analytics not being available.

This cleans them out.
I'd go with
```DELETE from art WHERE media_type='tvshow' AND NOT EXISTS (SELECT 1 FROM tvshow WHERE tvshow.idShow = art.media_id)```
and
```DELETE from art WHERE media_type='season' AND NOT EXISTS (SELECT 1 FROM seasons WHERE seasons.idSeason = art.media_id)```