drivers: net: xen-netfront: remove residual dead code

This patch removes some unused arrays from the netfront private
data structures. These arrays were used in "flip" receive mode.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>