Merge branch '381456-ff-cleanup' into 'master'

[Feature flag] Cleanup super_sidebar_nav feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/122480



Merged-by: Jose Ivan Vargas <jvargas@gitlab.com>
Approved-by: Lukas 'Eipi' Eipert <leipert@gitlab.com>
Approved-by: Joseph Wambua <jjoshua@gitlab.com>
Approved-by: Jarka Košanová <jarka@gitlab.com>
Approved-by: Jose Ivan Vargas <jvargas@gitlab.com>
Approved-by: Andrejs Cunskis <acunskis@gitlab.com>
Co-authored-by: Thomas Hutterer <thutterer@gitlab.com>