Remove `metrics_dashboard_exhaustive_validations` feature flag & feature

The metrics_dashboard_exhaustive_validations feature flag was introduced for the Metrics Dashboard, but was only ever enabled for gitlab internal projects.

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

Implementation Guide

• Delete relevant feature flag: config/feature_flags/development/metrics_dashboard_exhaustive_validations.yml
• Search codebase for references to metrics_dashboard_exhaustive_validations & delete usages of flag
• Delete any code that is now "dead" because the flag is removed (look at Gitlab::Metrics::Dashboard::Validator.errors method)
• Ping a GitLab team member to delete the flag from the gitlab.com & staging environments