Delete or rename files which are no longer needed

These were from when we had the `buckets` and `users` collections in
Mongo. They don't make sense any more.
Don't change metadata in capped/uncapped migration

We can't set metadata about data-sets any more as it lives in
Stagecraft.
Merge pull request #289 from alphagov/remove-old-buckets-legacy

Remove references to buckets