Remove filter_deleted_groups feature flag
What does this MR do and why?
The feature flag is fully rolled out on gitlab.com.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
How to set up and validate locally
See !158309 (merged).
Related to #470628 (closed)