Remove unnecessary md header divider when previewing

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149012



Merged-by: Phil Hughes <me@iamphill.com>
Approved-by: Jeremy Elder <jelder@gitlab.com>
Approved-by: Sascha Eggenberger <seggenberger@gitlab.com>
Approved-by: Diana Zubova <dzubova@gitlab.com>
Approved-by: Phil Hughes <me@iamphill.com>
Co-authored-by: Chad Lavimoniere <clavimoniere@gitlab.com>