videodb: add update/cleanup logic for version 91 in case of duplicate actors

This is a fix for the videodb update procedure introduced in #5653. There seem to be videodb's out there with multiple entries in the `actors` table that have the exact same name. I have no idea how that is possible because our code basically prevents that. A possibility would be an external tool that writes directly into our database (@MilhouseVH: any chance you are or have been using such a tool?).

I tested this with manually messing up my database and then updating from version 90 to 91 and it worked fine. But it's terribly slow (several seconds on my i7 with SQLite) because the `actors` table is so huge. So this will significantly slow down first start after upgrading from Helix to I**** for every user.

The question is whether we should/need to do the same thing for the `country`, `genre`, `studio` and `tag` tables.