Remove peek-sidekiq

What does this MR do?

It removes peek-sidekiq gem. The reason for removing this gem is that it's not being maintained anymore and it uses alias_method_chain which is deprecated in rails 5 (and removed in 5.1) which throws a deprecation warning in rails 5 CI job (https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/96869550), the issue is pending upstream (including a fix) - https://github.com/suranyami/peek-sidekiq/issues/3 for a while but it has never been fixed.

Peek-sidekiq is used only in performance bar for displaying sidekiq statistics.

An alternative to removing this gem would be fork&maintain it ourselves or monkey patch the alias_method_chain, but I don't incline to any of these options.

What are the relevant issue numbers?

Closes #51407 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides