Remove exempt_paid_namespace_members_...from_identity_verification FF

What does this MR do and why?

• Resolves Clean up exempt_paid_namespace_members_and_enterprise_users_from_identity_verification FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/434810

Remove exempt_paid_namespace_members_and_enterprise_users_from_identity_verification feature flag (FF) which has been enabled in production for 3 months.

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