Simplify header.scss

After !138918 (merged) is merged, the main legacy nav topbar is gone. There are effectively just two remaining partials which use classes like .navbar-gitlab, .navbar-nav and .navbar-sub-nav:

• app/views/layouts/header/_empty.html.haml
• app/views/layouts/header/_super_sidebar_logged_out.haml

We probably can simplify the code quite a bit.