mac802154: llsec: add forgotten list_del_rcu in key removal

During key removal, the key object is freed, but not taken out of the
llsec key list properly. Fix that.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>