Remove unused metrics dashboard helpers

What does this MR do and why?

Changes: Removes spec/support/helpers/metrics_dashboard_helpers.rb, which is no longer used.

Context: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features. 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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