Remove `disable_metric_dashboard_refresh_rate ` feature flag & feature

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.

Implementation Guide

• Delete relevant feature flag: config/feature_flags/development/disable_metric_dashboard_refresh_rate.yml
• Search codebase for references to disableMetricDashboardRefreshRate & delete usages of flag from frontend
• Search codebase for references to disable_metric_dashboard_refresh_rate & delete usages of flag backend
• Delete any code that is now "dead" because the flag is removed (look at app/assets/javascripts/monitoring/components/refresh_button.vue)
• Ping a GitLab team member to delete the flag from the gitlab.com & staging environments