#214581 Delete star from metrics dashboard endpoint

What does this MR do?

It implements final backend piece for #214581 (closed) by adding delete endpoint to /projects/:id/metrics/user_starred_dashboards API resource.

Add paths to new API endpoint into metrics_dashboard response.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content