rtnetlink: remove IFLA_BOND_SLAVE definition

This is in net-next only, for couple of days. Not used anymore, and never
should have been. So just remove it and pretend it was never there.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>