xfs: remove unused fields from struct xbtree_ifakeroot

Remove these unused fields since nobody uses them.  They should have
been removed years ago in a different cleanup series from Christoph
Hellwig.

Fixes: daf83964 ("xfs: move the per-fork nextents fields into struct xfs_ifork")
Fixes: f7e67b20

 ("xfs: move the fork format fields into struct xfs_ifork")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>