Never run startup css in jh

We don't want to run the startup-css-check job for JH because it's preferable to just inherit what we get from the main project. See https://gitlab.com/gitlab-org/gitlab/-/issues/331812#note_653073738 for more content.

Does it work?

After adding in the changes from https://gitlab.com/gitlab-jh/gitlab/-/merge_requests/67 which is currently failing for startup-css-check, you'll notice that the job isn't included anymore: