Do not check Authorization header in varnish

This is checked in backdrop and backdrop returns a helpfull error
message.

Also, less logic in varnish FTW!

[Delivers #69911218]
https://www.pivotaltracker.com/story/show/69911218
Merge pull request #398 from alphagov/do-not-check-auth-in-varnish

Do not check Authorization header in varnish