Remove redundant code

We now use the general purpose PublishingApiWorker for publishing all the things
and this Edition-specific one is no longer used.
Unscheduling editions with Publishing API

When a scheduled edition is unscheduled, we remove any publish intents that are
registered for it with Publishing API. If it is a first-edition, we also replace
the 'coming_soon' content item that will have been published when it was
scheduled with a 'gone' item.