Remove arkose_labs_oauth_signup_challenge feature flag

What does this MR do and why?

• Resolves Clean up arkose_labs_oauth_signup_challenge FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: #388033 (closed)

Remove arkose_labs_oauth_signup_challenge feature flag (FF) which has been enabled in production for 10 months. This feature can still be toggled with identity_verification FF for now but that FF is planned to be removed soon as well.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.