Merge branch '442218-remove-deprecated-route' into 'master'

Remove deprecated preview_markdown routes

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/156070



Merged-by: Piotr Skorupa <pskorupa@gitlab.com>
Approved-by: Carla Drago <cdrago@gitlab.com>
Approved-by: Piotr Skorupa <pskorupa@gitlab.com>
Co-authored-by: Mario Celi <mcelicalderon@gitlab.com>