Optimize Group#for_epics scope

What does this MR do?

Remove optimized_groups_user_can_read_epics_method feature flag and keep the optimized code in the codebase. The FF was enabled in 12.6 and no issues were reported.

Performance comparison: !20833 (comment 261878280)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #37748 (closed) Closes #37368 (closed)