Clean metric templates yml related files

What does this MR do and why?

Aids in removing/updating files related to #397141 (closed)

Updates specs and usages

Screenshots or screen recordings

NA

How to set up and validate locally

Everything should just work as is and pipeline should be 💚

1. The metric YML files should load as is even after deprecating and removing the Metric blob viewer in this MR.
2. The list should not show metrics-dashboard.yml in the template type dropdown.
3. Specs of the relevant code that handle metrics dashboard yml files view is removed, hence the specs are also removed. Rest of the functionality regarding Apply a template should work as expected.
4. API call to /projects/<project_id>/templates/metrics_dashboard_ymls/Default should return bad request as same as other unknown template types.

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 #397141 (closed)