Clean up arkose_labs_phone_verification_challenge FF

What does this MR do and why?

• Resolves Clean up arkose_labs_phone_verification_challenge FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: #432747 (closed)

Remove arkose_labs_phone_verification_challenge feature flag (FF) and code used by the feature.

This feature was made obsolete with the introduction of Require Arkose challenge before send phone code... (!152335 - merged).

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