Revert the temporary patch to work around Snow Leopard readdir() bug.

Revert the temporary patch to work around Snow Leopard readdir() bug.
Apple has fixed that bug in 10.6.2, and we should encourage users to
update to that version rather than trusting this cosmetic patch.
As was recently noted by Stephen Tyler, this patch was only masking
the problem in the context of DROP TABLESPACE, but the failure could
occur in other places such as pg_xlog cleanup.