Remove support of LUN-based CD changers from cd(4) driver.

This code was heavily broken few months ago during CAM locking changes.
Fixing it would require almost complete rewrite. Since there are no
known devices on market using this interface younger then ~15 years, and
they are CD, not even DVD, I don't see much reason to rewrite it.

This change does not mean those devices won't work. They will just work
slower due to inefficient disks load/unload schedule if several LUNs
accessed same time.

Discussed with: ken@
Silence on: scsi@, hardware@
MFC after: 1 week