xfs: cull repair code that will never get used

These two functions date from the era when I thought that we could
rebuild btrees by creating an alternate root and adding records one by
one.  In other words, they predate the btree bulk loader.  They're not
necessary now, so remove them.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>