audit: Delete va_copy kludge

When I developed fix c3d4c109305b2db8a63b754c1894ad37326dc340 I used
va_copy.  I was nervous because it required C99, but I had no
problem with any of the buildbots, nor did any reviewer comment.
audit/audit.c contains a local hack to simulate va_copy in the
pre C99 days.  There are no uses of va_copy in audit.c but
presumably at some point there was.  Delete the local va_copy.

Change-Id: I5e30c7e3052aeffe56e366888c5a3db3a705fd00
Reviewed-on: http://gerrit.openafs.org/11184
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>