Remove old metrics dashboard schema validation code

Background

To enable more in depth validation of metrics dashboard schemas, we switched to new implementation of metrics dashboard schema validation at !38925 (merged)

What needs to be done

After the change there is some left over dead code withing files at https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/models/performance_monitoring and https://gitlab.com/gitlab-org/gitlab/-/tree/master/spec/models/performance_monitoring