Remove identity_verification_auto_ban feature flag

What does this MR do and why?

• Resolves Clean up identity_verification_auto_ban FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/417557

Remove identity_verification_auto_ban feature flag (FF) which is enabled by default in production for 7 months.

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