Remove abandoned checkpoint/poe plugin.

From the comment block in checkpoint_poe.c:

  This is based upon checkpoint support of poe in the 2005 time frame for
  the ASCI Purple computer. It does not work with current versions of POE.
  From Gary Mincher (IBM, Sept 6 2012): "Checkpoint/restart on Linux is only
  supported for user-space parallel jobs with a maximum of 512 tasks that are
  run on Power 775 nodes, but jobs that use a resource manager or scheduler
  other than LoadLeveler are not supported."
N