Drop worker_resource_boundary :cpu from AutoMergeProcessWorker and MergeTrains::RefreshWorker

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The following discussion from !51433 (merged) should be addressed:

• @reprazent started a discussion: (+3 comments)

  How did you decide the resource boundary of this worker? Since the worker isn't running yet, we don't really have information about it: https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#determining-whether-a-worker-is-cpu-bound

  Could this start by running on the catchall shard to begin with?