Clean up arkose_labs_signup_data_exchange feature flag

What does this MR do and why?

• Resolves Clean up arkose_labs_signup_data_exchange FF task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/488
• Rollout issue: #435275 (closed)

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

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