Remove unused assignments, noticed by Clang analyzer.

MFC after:	1 week
Avoid panic in ofwbus caused by not released resource list entry

After resource allocation and release, resource list entry
stays non-NULL. This causes panic in ofwbus_alloc_resource()
on subsequent resource allocation.
Clean appropriate list entry on release to avoid this.

Obtained from:	Semihalf
Reviewed by:	ian
Sponsored by:	The FreeBSD Foundation