Remove the `mr_approvers_filter_hidden_users` feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183058



Merged-by: Allen Cook <acook@gitlab.com>
Approved-by: Amy Qualls <aqualls@gitlab.com>
Approved-by: Shola Quadri <squadri@gitlab.com>
Approved-by: Allen Cook <acook@gitlab.com>
Co-authored-by: Phil Hughes <me@iamphill.com>