btrfs: remove unused btrfs_dio_private::private

We haven't used this since commit 9be3395b ("Btrfs: use a btrfs
bioset instead of abusing bio internals").
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/btrfs_inode.h

@@ -301,7 +301,6 @@ struct btrfs_dio_private {
 	u64 logical_offset;
 	u64 disk_bytenr;
 	u64 bytes;
-	void *private;
 
 	/* number of bios pending for this dio */
 	atomic_t pending_bios;

fs/btrfs/inode.c

@@ -7923,7 +7923,6 @@ static struct btrfs_dio_private *btrfs_create_dio_private(struct bio *dio_bio,
 	bio->bi_private = dip;
 	btrfs_io_bio(bio)->logical = file_offset;
 
-	dip->private = dio_bio->bi_private;
 	dip->inode = inode;
 	dip->logical_offset = file_offset;
 	dip->bytes = dio_bio->bi_iter.bi_size;