LU-4396 tests: Remove unnecessary skip in sanity test_31n

LU-4396 tests: Remove unnecessary skip in sanity test_31n

Since test_31n now uses the function free_fd to find an unused file
descriptor, it no longer depends on fd 173 being free.  This change
removes that test on whether fd 173 is in use.

Signed-off-by: Ryan Haasken <haasken@cray.com>
Change-Id: I3b68e278d4f3a8af42e57b073ccbe45b80e76ecd
Reviewed-on: http://review.whamcloud.com/8622
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>