Fix duplicate entry of line-height

@iamphill I see you created the source/stylesheets/_sandwich-page-layout.scss. There are two entries of line-height, I'm not sure what's the right one.

scss-lint was updated in https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/4692/diffs#21fb5836b499a2be648386aac055d2e069160d6c_131_129 so this was failing https://gitlab.com/gitlab-com/www-gitlab-com/builds/9367944.