Remove unused prometheus metric service

What does this MR do and why?

Changes: Removes Projects::Prometheus::Metrics::DestroyService & supporting BaseService, which are unused.

Context: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features. The Metrics Dashboard & Prometheus Metrics/Alerts (used for monitoring apps deploying using gitlab) were deprecated in 14.7 and removed from the product in 16.0.

• grouprespond related issue: Metrics: Delete models & database tables/columns (#397142)
• Monitor:Metrics removal epic: Monitor: Metrics Deprecation and Removal (&10107)
• Reference MR with all related removals: Draft: Remove all remaining metrics dashboard-r... (!160325 - closed)
• Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

The features related to this removal were disabled in %16.0, and have been partially deleted since then. So there's nothing really to test here.