[iwm] remove dead code.
if_iwm - GC some dead code, left by a partially applied OpenBSD change.
Taken-From: OpenBSD (if_iwm.c r1.69)
Submitted by: Imre Vadasz <imre@vdsz.com>
Obtained from: DragonflyBSD git 07dfed32ea39b980b0b80d27ff938e7c3ca4c0b5