net/xen-netback: Remove unused code in xenvif_rx_action

net/xen-netback: Remove unused code in xenvif_rx_action

The variables old_req_cons and ring_slots_used are assigned but never
used since commit 1650d5455bd2dc6b5ee134bd6fc1a3236c266b5b "xen-netback:
always fully coalesce guest Rx packets".

Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 44f0764cfec9c607d43cad6a51e8592c7b2b9b84)
Signed-off-by: Annie Li <annie.li@oracle.com>