documentation: Update ide-cd documentation to reflect CONFIG_BLK_DEV_HD_IDE removal

CONFIG_BLK_DEV_HD_IDE was removed in commit 80aa31cb ("ide:
remove CONFIG_BLK_DEV_HD_IDE config option (take 2)") but the ide-cd
documentation was not updated and still asks users to disable it,
which is misleading and involves a fruitless search.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>