Remove ProjectMetricsSetting model

What does this MR do and why?

Remove ProjectMetricsSetting model

Remove specs and usages of the model

Remove association in project model

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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) and #397142