Eliminate pointless changes between nbsd50/60 params

param.nbsd60.h removed AFS_64BIT_ENV and added an empty expression
to an #if.

AFS_64BIT_ENV is required for any platform to build (on 1.6)
This is a 1.6-only change. On master, AFS_64BIT_ENV was removed entirely
in commit fc9aa428.

Change-Id: I57d7e2a2ef5ab4bf787a99083d35ba70e240710c
Reviewed-on: http://gerrit.openafs.org/10137
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jonathan A. Kollasch <jakllsch@kollasch.net>
Tested-by: Jonathan A. Kollasch <jakllsch@kollasch.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>