Remove `project_saved_replies_flag`

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175646



Merged-by: Aaron Huntsman <ahuntsman@gitlab.com>
Approved-by: Shane Maglangit <smaglangit@gitlab.com>
Approved-by: Amy Qualls <aqualls@gitlab.com>
Approved-by: Aaron Huntsman <ahuntsman@gitlab.com>
Co-authored-by: Thomas Randolph <trandolph@gitlab.com>