Remove the prefix_ci_build_tokens feature flag

Closes gitlab-com/gl-infra/production#17299

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142622



Merged-by: Eugie Limpin <elimpin@gitlab.com>
Approved-by: Hordur Freyr Yngvason <hfyngvason@gitlab.com>
Reviewed-by: Nick Malcolm <nmalcolm@gitlab.com>
Co-authored-by: Nick Malcolm <nmalcolm@gitlab.com>