[Feature flag] Cleanup super_sidebar_nav feature flag

What does this MR do and why?

Removes the super_sidebar_nav feature flag and its usage in the codebase.

The FF was set to default_enabled: true for %16.0 and can be removed now.

Screenshots or screen recordings

No changes.

How to set up and validate locally

There shouldn't be any changes. Just verify that the New navigation toggle (in your user dropdown) still works and you can toggle between old and new nav.

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 #381456 (closed)