Remove arkose_labs_trial_signup_challenge feature flag

What does this MR do and why?

• Resolves Clean up arkose_labs_trial_signup_challenge FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/395754

Remove arkose_labs_trial_signup_challenge feature flag (FF) which has been enabled in production for 10 months. For now, this feature can still be toggled with arkose_labs_signup_challenge FF but that will also be removed in favor of just using identity_verification FF.

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