Clean up remove_cross_join_from_vulnerabilities_projects_grade feature flag

What does this MR do and why?

Cleans up the feature flag for remove_cross_join_from_vulnerabilities_projects_grade and keeps existing code.

Closes #508631 (closed).

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.