Cleanup unused issue workers

What does this MR do and why?

This MR is the third and last step in a trio of MRs !72802 (merged), !73423 (merged), !72803 aimed to fixing cop offences for Gitlab/NamespacedClass #321982 for issue workers.

• First MR introduced the new workers and queues.
• Second MR schedules new jobs through the new workers
• This last MR cleans up old workers a couple milestones later, so that any jobs queued to old workers can finish the work.

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