Merge branch 'ngala/remove-ff-pages-use-open-file' into 'master'

Cleanup feature flag ff_pages_use_open_file

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/180926



Merged-by: Jonas Larsen <jlarsen@gitlab.com>
Approved-by: Jayakrishnan Mallissery <jmallissery@gitlab.com>
Approved-by: Jonas Larsen <jlarsen@gitlab.com>
Co-authored-by: ngala <ngala@gitlab.com>