Remove unused metrics dashboard templates
What does this MR do and why?
Changes: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features.
Scope: Everything under lib/gitlab/metrics/templates/. These templates were originally made available as options for users to create new metrics dashboard yml files. That behavior has been removed.
Context: The Metrics Dashboard (used for monitoring apps deploying using gitlab) was deprecated in 14.7 and removed from the product in 16.0. There are many more files to remove related to this cleanup, but this should be a self-contained chunk.
- 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)
- Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838)
MR acceptance checklist
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How to set up and validate locally
These features were disabled in %16.0 behind the :remove_monitor_metrics feature flag, and have been partially deleted since then. So there's nothing really to test here. It's no longer possible to set up these features.