Metrics: Delete sidekiq worker classes

Overview

⚠ Removal should take place in %16.1 or later, at least one milestone after Metrics: Cleanup background workers & importers (#397141 - closed).

⚠ Category:Metrics code is intertwined with the metrics GitLab reports on itself. Only Category:Metrics should be removed. Similarly, some items named like PrometheusAlert are also used for AlertManagement::Alert and should not be removed.

As part of Monitor: Metrics Deprecation and Removal (&10107), we want to delete the background processing tasks for Category:Metrics. This issue is to delete the worker classes nullified in #397141 (closed).

Relevant processes to follow

• Guide for removing worker classes
• Updating documentation for sidekiq queues