Resolve "Remove the method override for `perform` in WaitableWorker"

What does this MR do and why?

WaitableWorker is a concern that has not been made use of in workers, since quite a while now. It only has the perform method in it now, the methods like bulk_perform_and_wait has been removed from it a long while ago. Hence, now in 16.0, it is safe to remove this concern altogether.

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Related to #367224 (closed)