Cleanup old constant references

**WARNING: Do not merge until https://github.com/alphagov/whitehall/pull/1794 has been deployed and given enough time to run down the job queue**

Once the code that renames and reworkes these workers has been deployed and the queue has had adequate time to run down, we can get rid of these temporary references to the old workers.

* [x] Peer review
* [x] https://github.com/alphagov/whitehall/pull/1794 has been deployed
* [x] The sidekiq monitor has been checked to ensure there are no longer any `PublishingApiRegisterArtefactWorker` or `PublishingApiRegisterOrganisationWorker` jobs remaining