Remove `custom_roles_on_groups` feature flag

What does this MR do and why?

• Added in 16.3: !125652 (merged)
• Default enabled since 16.3: !129211 (merged)
• !129211 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.