[#49119857] Remove dupe 5xx handling in Varnish
[#49119857] Remove dupe 5xx handling in Varnish

Remove the 5xx handling which served from stale. This hasn't been hit since
the zero-TTL `hit_for_pass` was added further up. I'm pretty sure this wasn't
working anyway, since it only restarts if `req.restarts` is greater than zero,
which it won't be when we don't call `restart` anywhere else.

I think there's some benefit to what this should have been doing; which is
to protect misbehaving apps from being hammered with requests. However it needs
more thought as to how it will interact 5xx caching and Licensing's ability to
send it's own error pages.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet