LU-1199 build: Assume that ldiskfs PDO is now standard

PDO (Parallel Directory Operations) is now a standard ldiskfs feature.
The old patch series files that lacked PDO support have been removed
from the tree, so now we can drop all of the places that conditionally
use PDO, and just assume that PDO is standard.

Also remove support for rhel5 series file in ldiskfs, since that
no longer exists in the tree.

Change-Id: I76b5b31a4614c1ade3b4b5890a111cdabc4afa63
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/5004
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>