[SCSI] sr: remove dead code

This patch fixes an issue reported in drivers/scsi/sr.c by Coverity

Error reported: Pointer returned from "scsi_cd" is never used

Patch description:
 Remove the scsi_cd() call as it does not have any effect.

Signed-off-by: Jayachandran C. <c.jayachandran@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>