Remove PipelineSuccessWorker

Currently, PipelineSuccessWorker is no-oped due to the https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28780 change. If we don't have a plan to use this worker for any other purposes, we should remove it completely.

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Follow-up from "Introduce sidekiq worker for auto merge process"

The following discussion from gitlab-ce!28780 should be addressed:

• @tkuah commented on a discussion: (+3 comments)

  Can we please add a code comment here why this was no-op'ped ?

  If possible, we should aim to remove workers if un-used as that will be one less item in the Sidekiq queue. Perhaps, we can aim to remove this worker as a gitlab-ce~3011693 issue