Delete Sidekiq queues list from Redis

Quoting @cmiskell from Slack:

Huh, TIL that Sidekiq maintains a set in Redis called 'queues' (with the prefix for us that's resque:gitlab:queues). It's literally a set of all queue names Sidekiq has ever seen (sadd called when the client pushes a job to the queue), but it's only used for Stats (and by gitlab-exporter for sidekiq_queue_size) to get a list of queues to iterate over getting lengths for (as opposed to doing a keyscan, which would be problematic and slow). But nothing automatic cleans it up (ever) so there's just on 500 entries right now, and all the queues we've stopped using during this project (or haven't used for years because we moved/renamed them) still have an entry in that set, and when we query stats we faithfully go and look to see if there are any in those queues (which there never are). The exporter exports a metric for each of them, pinned forever more to 0 for (now) most of them.

I think we should, as a matter of cleanliness at the end of this project, either completely nuke that set and let it be rebuilt (quick, should be safe, but feels rough), or clean it up more carefully (iterate over entries, delete them if they're not in the very small list of known active queues). If we over-delete and the queues are in active use, it'll get fixed automatically.