Remove metrics dashboard concern and usage

What does this MR do and why?

The metrics dashboard concern is not used anywhere else, hence we can remove it.

Also removes specs for the concern.

Screenshots or screen recordings

NA

How to set up and validate locally

Since this was unused code, everything else should just work, I relied on pipeline for this.

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 #397138 (closed)