xfs: remove unused struct declarations

After scratching my head looking for "xfs_busy_extent" I realized
it's not used; it's xfs_extent_busy, and the declaration for the
other name is bogus.  Remove that and a few others as well.

(struct xfs_log_callback is used, but the 2nd declaration is
unnecessary).

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>