Merge branch 'ia-cleanup-duplicate_phone_number_assume_high_risk' into 'master'

Cleanup the duplicate_phone_number_assume_high_risk feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/170508



Merged-by: Aaron Huntsman <ahuntsman@gitlab.com>
Approved-by: Eugie Limpin <elimpin@gitlab.com>
Approved-by: Aaron Huntsman <ahuntsman@gitlab.com>
Co-authored-by: imand3r <ianderson@gitlab.com>