btrfs: remove btrfs_bio_end_io_t

The btrfs_bio_end_io_t typedef was introduced with commit
a1d3c478 ("btrfs: btrfs_multi_bio replaced with btrfs_bio")
but never used anywhere. This commit also introduced a forward declaration
of 'struct btrfs_bio' which is only needed for btrfs_bio_end_io_t.

Remove both as they're not needed anywhere.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.h

@@ -300,9 +300,6 @@ struct btrfs_bio_stripe {
 	u64 length; /* only used for discard mappings */
 };
 
-struct btrfs_bio;
-typedef void (btrfs_bio_end_io_t) (struct btrfs_bio *bio, int err);
-
 struct btrfs_bio {
 	refcount_t refs;
 	atomic_t stripes_pending;