Clean up phone_verification_for_low_risk_users experiment

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/665

This MR removes the code for the phone_verification_for_low_risk_users experiment now that the replacement implementation (see https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/32) has been completed.

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