Delete http_status columns from mappings tables

This needs to be deployed after https://github.com/alphagov/transition/tree/release_543 and then https://github.com/alphagov/bouncer/pull/71, if we're confident that everything is working correctly. The remaining cleanup steps then should be:
- stop setting `mapping.http_status` in Bouncer's specs
- https://github.com/alphagov/transition/pull/287
- simplify the versions helpers to only handle `type` and not `http_status` in changesets