virtio_net: remove send queue

Now we have a virtio detach API (in commit
f9bfbebf34eab707b065116cdc9699d25ba4252a), we don't need to track xmit
skbs in the virio_net driver, which improves transmission performance.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>