xen-netfront: remove dead code

This patch removes some residual dead code left over from removing the
"flip" receive mode.  This patch doesn't change the generated output
at all, since gcc already realized it was dead.

This resolves the "regression" reported by Adrian.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Adrian Bunk <bunk@stusta.de>
Cc: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>