Drop 'gateway' directive from campus host interfaces

BusyBox ifup translates 'gateway X' into 'ip route add default via X'
and aborts the whole ifup run with RC=1 when that command fails with
'File exists' — which always happens on first boot because the docker
management bridge has already installed its own default via eth0. As a
result the 'post-up ip route replace default' was never executed and
the host kept the management default.

Remove the 'gateway' line so ifup only runs the idempotent 'post-up ip
route replace default via <fabric-gw>' and the fabric default wins.

hosts/campus-host2_interfaces

@@ -5,5 +5,4 @@ auto eth1
 iface eth1 inet static
     address 10.60.70.102
     netmask 255.255.255.0
-    gateway 10.60.70.1
     post-up ip route replace default via 10.60.70.1