Simplify header CSS by using govuk_template defaults

Lots of previously-specifically-whitehall CSS is now just the default, as doesn't need overrided here.

* Remove custom whitehall bar and use the standard gov.uk bar
* Remove custom whitehall colours, overrides. These have been migrated to
* Specify border-bottom, as that's usually on wrapper, which whitehall doesn't have
* Remove custom dept styling CSS and class hooks, which wasn't actually be applied anywhere

Was originally discussed in https://github.com/alphagov/whitehall/pull/1489, created a new PR as the previous one changed a lot during discussion.

Needs to be deployed along with or after https://github.com/alphagov/static/pull/411 (already merged)