md: remove mddev_to_conf "helper" macro

Having a macro just to cast a void* isn't really helpful.
I would must rather see that we are simply de-referencing ->private,
than have to know what the macro does.

So open code the macro everywhere and remove the pointless cast.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid0.h

@@ -17,6 +17,4 @@ struct raid0_private_data
 
 typedef struct raid0_private_data raid0_conf_t;
 
-#define mddev_to_conf(mddev) ((raid0_conf_t *) mddev->private)
-
 #endif