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**

Now that the code that renamed and reworked 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.

* [ ] https://github.com/alphagov/whitehall/pull/1794 has been deployed
* [ ] the sidekiq monitor has been checked to ensure there are no longer any `PublishingApiRegisterArtefactWorker` or `PublishingApiRegisterOrganisationWorker` jobs remaining