scsi_cd: Remove commented-out code

This is left over from before the implementation of asynchronous media
probing, which was committed in dd78f43259ef
("scsi_cd: make the media check asynchronous") quite a while ago now.

No functional change intended.

MFC after: 1 week
Reviewed by: imp, avg
Differential Revision: https://reviews.freebsd.org/D43650