Remove the job deduplication feature flag

We have a feature flag that allows disabling the automatic deduplication of sidekiq jobs for idempotent workers in DuplicateJob#droppable?.

The job deduplication has been enabled for a while without issue, we added the feature flag in #327 (closed), so I think we can now remove it.

Idempotent jobs that for some reason should not be deduplicated can now use the deduplicate :none strategy.