Merge branch 'cngo-delete-unused-header-code' into 'master'

Delete unused issuable header code

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/129414



Merged-by: Coung Ngo <cngo@gitlab.com>
Approved-by: David Dieulivol <ddieulivol@gitlab.com>
Approved-by: Martin Čavoj <mcavoj@gitlab.com>
Approved-by: Vitaly Slobodin <vslobodin@gitlab.com>
Approved-by: Drew Blessing <drew@gitlab.com>
Reviewed-by: Martin Čavoj <mcavoj@gitlab.com>