Remove optimize_issuable_non_archived_scope feature flag

What does this MR do and why?

The feature flag optimize_issuable_non_archived_scope has been enabled globally which was added in !214838 (merged)

References

Rollout issue #582682 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

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.