ide-cd: remove dead dsc_overlap setting

dsc_overlap setting is not useful anymore for ide-cd so remove
"dsc_overlap" /proc setting, disallow changing the setting through
HDIO_SET_NICE ioctl and cleanup ide_cd{rom_setup,_release}().

Cc: Borislav Petkov <petkovbb@googlemail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>