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>