net: Remove the deprecated 'host_net_add' and 'host_net_remove' HMP commands

They are deprecated since QEMU v2.10, and so far nobody complained that
these commands are still necessary for any reason - and since you can use
'netdev_add' and 'netdev_remove' instead, there also should not be any
real reason. Since they are also standing in the way for the upcoming
'vlan' clean-up, it's now time to remove them.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>