Remove Auditor Access to Group Runners Feature Flag

What does this MR do and why?

Remove the feature flag auditor_group_runner_access which controls auditor access to group runners.

Flag Issue: #368089 (closed)

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.