[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).
IMO one vector is enough as you only use it for the announcement iteration so maybe change it into an std::set<> (to avoid duplicates) and then fill it with all the IDs from both queries.