Remove arkose_labs_signup_challenge feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/147767



Merged-by: Eugie Limpin <elimpin@gitlab.com>
Approved-by: Kos Palchyk <kpalchyk@gitlab.com>
Approved-by: Ian Anderson <ianderson@gitlab.com>
Approved-by: charlie ablett <cablett@gitlab.com>
Approved-by: Illya Klymov <iklymov@gitlab.com>
Approved-by: Aboobacker MK <akarakath@gitlab.com>
Reviewed-by: Kos Palchyk <kpalchyk@gitlab.com>