Now that we have implemented unified short frames padding in the
QEMU networking codes, the small packet check logic in the receive
path is no longer needed.

Suggested-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>