net: Remove the legacy "-net channel" parameter

It has never been documented, so hardly anybody knows about this
parameter, and it is marked as deprecated since QEMU v2.6.
Time to let it go now.

Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>