ieee802154: hwsim: remove not used ifup list

This list is a leftover from fakelb driver which had always a full mesh
topology. Idea was to remember all phy's which are currently used by the
subsystem and deliver everything out. The hwsim driver works differently
each phy has a list of other phy's to deliver frames which allows a
own mesh topology.

Signed-off-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>