Comment out some pointless device open/close around reading device IDs.

FreeBSD ZFS port unlike OpenSolaris does not use device IDs, and does not
implement respective devid_*() fuctions.  It is pointless to open devices
just to close them back immediately.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.

(cherry picked from commit cfea4862aa9adab7aafc28b0ee56bfe6ce823d40)