af_ieee802154: drop IEEE802154_SIOC_ADD_SLAVE declaration

IEEE802154_SIOC_ADD_SLAVE was used to allocate 802.15.4 interfaces
on the top of radio. It's not used anymore, drop it.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>