Stop removing Expires header

reason:
 - Expires header is widely supported than max-age. e.g.: HTTP/1.0 clients.
 - max-age directive always overrides Expires setting under HTTP/1.1,
   so there is no need to remove Expires here.

slapos/recipe/generic_varnish/template/default.vcl.in

@@ -157,8 +157,6 @@ sub vcl_fetch {
     if (beresp.ttl > 0s) {
         /* Setup grace period for 30days for all cacheable contents */
         set beresp.grace = 30d;
-        /* Remove Expires from backend, it's not long enough */
-        unset beresp.http.expires;
         # /* Set the clients TTL on this object */
         # set beresp.http.cache-control = "max-age = 300";
         /* Set how long Varnish will keep it */