Cleanup compliance_group_dashboard FF

What does this MR do and why?

This MR removes compliance_group_dashboard feature flag as feature is now considered stable

References

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Run FILTER=compliance_report_data SEED_COMPLIANCE_REPORT_DATA=1 bundle exec rake db:seed_fu, it will output seeded group URL
2. Ensure you're on Ultimate license or if you are using GITLAB_SIMULATE_SAAS=1 grant Ultimate plan to new seeded group
3. Go to Secure > Compliance center. Observe overview tab

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #555804 (closed)