[SCSI] zfcp: remove zfcp_ccw_unregister function

Remove unused zfcp_ccw_unregister function (leftover from zfcp's
module_exit era).

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andreas Herrmann <aherrman@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>