ide-cd: remove struct atapi_{mechstat_header,changer_info,slot}

* Remove ->changer_info from struct cdrom_info (it is never allocated).

* Remove unused struct atapi_{mechstat_header,changer_info,slot}.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>