varnish: remove cache separation for HHVM

* We remove completely the X-Use-HHVM header
* Do not vary on it anymore
* We just send the first request to hhvm if the hhvm=true cookie is set
* We don't fallback on zend anymore.

Change-Id: Ic92256cc625a42ab993815a2a99ec6b3038cc298
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>