[videodb] Fix MySQL error while cleaning tvshows

This fixes the MySQL failure discussed [here](https://github.com/xbmc/xbmc/pull/5145#issuecomment-51697600).

The existing DELETE statement is not valid in MySQL, and will fail whenever an entire tvshow folder is removed during a clean.

The solution is to iterate over a vector of tvshow id's, which will work for both MySQL and SQLite.

As a bonus, the removed tvshow id's are now announced correctly (they weren't announced at all previously).
You're right, although I don't think a set is necessary as there should be no overlap in the two queries - any tvshow deleted by the first query won't be in the second. I could actually use a single string field too, clearing it before second query, it's just that I tried to keep the code fairly consistent with what already existed.