This rolls back the fix for #1206 -- the decision is made regardless
of the pass state of the backend fetch. So delivery sets resp.status
to 304 at ESI level 0 iff the conditions in req.http.{INM,IMS} and
resp.http.{ETag,Last-Modified} allow it. To restore the behavior
from #1206, remove the If-* headers in vcl_pass.

Fixes #2042