Fully deprecate legacy sidekiq workers for onboarding

What does this MR do and why?

Fully deprecate legacy sidekiq workers for onboarding

• no longer needed as we transitioned to ee/workers/onboarding/progress_tracking_worker.rb
• following https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#in-a-subsequent-separate-minor-release
  • workers were no-oped in %17.4

Changelog: other

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Related to #472664 (closed)