Remove unused metrics dashboard annotation policy

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: Any unused permissions/policy code related to Metrics::Dashboard::Annotations or Metrics::UserStarredDashboards (both classes now 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 - closed)
• Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838 - closed)

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