Revert "network: disable proxy_arp"

Proxy ARP apparently is still needed for HVMs.
This reverts commit fa8b05a83c7afa9759227e67c45ede1aed77e5b1.

Fixes QubesOS/qubes-issues#1421

network/vif-route-qubes

@@ -29,6 +29,7 @@ lockfile=/var/run/xen-hotplug/vif-lock
 case "$command" in
 	online)
 		ifconfig ${vif} up
+		echo 1 >/proc/sys/net/ipv4/conf/${vif}/proxy_arp
 		ipcmd='add'
 		iptables_cmd='-I PREROUTING 1'
 		cmdprefix=''