[Feature flag] Cleanup `super_sidebar_peek` feature flag

What does this MR do and why?

Remove super_sidebar_peek feature flag usage.

The feature flag was set to default_enabled: true in %16.0 !119706 (merged) and can be removed now.

Screenshots or screen recordings

No changes.

How to set up and validate locally

There shouldn't be any changes. Verify that moving the mouse to the left-edge of the viewport when the new navigation is collapsed still opens as expected.

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.

Related to #404966 (closed)