md: remove some dead fields from mddev_s

These fields have never been used.
commit 4b6d287f627b5fb6a49f78f9e81649ff98c62bb7
added them, but also added identical files to bitmap_super_s,
and only used the latter.

So remove these unused fields.

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

drivers/md/md.h

@@ -279,9 +279,6 @@ struct mddev_s
 	atomic_t			writes_pending; 
 	struct request_queue		*queue;	/* for plugging ... */
 
-	atomic_t                        write_behind; /* outstanding async IO */
-	unsigned int                    max_write_behind; /* 0 = sync */
-
 	struct bitmap                   *bitmap; /* the bitmap for the device */
 	struct {
 		struct file		*file; /* the bitmap file */