Merge branch '454861-clean-up-duo-pro-trials-for-free-plans' into 'master'

Clean up duo_pro_trials_for_free_plans feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149743



Merged-by: Felipe Artur <fcardozo@gitlab.com>
Approved-by: Minahil Nichols <minahilnichols@gitlab.com>
Approved-by: Felipe Artur <fcardozo@gitlab.com>
Co-authored-by: Serhii Yarynovskyi <syarynovskyi@gitlab.com>