Remove super_sidebar_nav_enrolled feature flag

What does this MR do and why?

Remove super_sidebar_nav_enrolled feature flag

The feature flag was used to determine whether a user should get the new nav if they haven't set a user preference yet. It has been default enabled since 16.1.

Changelog: other

See #410121 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Create a new user
2. Log-in as the new user and they should see the super sidebar

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.