Remove `pruneup' optimization.

This breaks on NTFS mounts, because they do not obey the usual
semantics for link counts (I've seen files with link count 0,
directories with link count 1, and - particularly relevant -
directories with subdirectories but link count 2).

IMO the real bug here is actually in the NTFS support, but this
`optimization' doesn't actually help much except in `impossible' cases
(and it's actually wrong in most of those), so it should probably be
removed for the sake of robustness anyway.