Removing 64-bit workaround text from chpldirent.h

When first implementing chpldirent.h as a helper header
in the test/studies/filerator/ directory, I had to put
in a bunch of #ifdef logic to mimic what was happening
on Crays in order to get it working with all PrgEnv's.
It seems to be that this was related to the #define of
_FILE_OFFSET_BITS in sys_basic.h, and that now that
that's commented out, this logic is no longer needed,
so I'm removing it.

I tested this by testing the filerator module on Cray HW
with the four Cray PrgEnvs.  The only failures were the
globberator.chpl runs, which seem to fail even without
the change.