Clean up auto_request_phone_number_verification_exemption feature flag

What does this MR do and why?

• Resolves Clean up auto_request_phone_number_verification_exemption FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: gitlab-com/gl-infra/production#17477 (closed)

Remove auto_request_phone_number_verification_exemption feature flag (FF) which has been enabled in production for 2 months.

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