Remove duplicated sections from sidekiq-cluster docs

What does this MR do?

Both negation (https://docs.gitlab.com/ee/administration/operations/extra_sidekiq_processes.html#negating-settings) and concurrency (https://docs.gitlab.com/ee/administration/operations/extra_sidekiq_processes.html#limiting-concurrency) are mentioned further up, and those sections are more useful as they describe how you configure GitLab. We don't expect people to be running this command manually.

!20397 (merged) also touched this lately, so the sections I kept are more up to date.

Related issues

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