Remove `disable_metric_dashboard_refresh_rate ` feature flag & feature

What does this MR do and why?

Related to #383126 (closed)

The disable_metric_dashboard_refresh_rate feature flag was introduced for the Metrics Dashboard, but never enabled for GitLab.com or self-managed users.

As the Metrics Dashboard feature is scheduled for removal in %16.0, we should remove the code related to the disable_metric_dashboard_refresh_rate feature flag and the flag itself.

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Related to #383126 (closed)