virtio_net: remove hcpu from virtnet_clean_affinity

The variable is never used.

CC: Michael S. Tsirkin <mst@redhat.com>
CC: Jason Wang <jasowang@redhat.com>
CC: virtualization@lists.linux-foundation.org
CC: netdev@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>