Delete CloneDashboardService and DashboardsController

What does this MR do and why?

Delete CloneDashboardService and DashboardsController

Delete and update related specs

Helps in moving #397139 (closed) forward with deletion.

Screenshots or screen recordings

NA

How to set up and validate locally

1. Pipeline should be 🍏
2. Dashboards controller returned not_found before due to remove_monitor_metrics FF. Deleting this controller will not change the functionality.
3. CloneDashboardService should not be shown in finder or any other service.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #397139 (closed)