Remove the backend deref code that is no longer needed by random/round-robin
directors, since they only reference other directors in the same VCL.

Stop messing with sp->vbe behind the back, make the assignment explicit
where it matters.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4414 d4fa192b-c00b-0410-8231-f00ffab90ce4