delete unused constant

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1922 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
handle when filetype == MPI_DATATYPE_NULL

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1923 c368ceb8-fd28-0410-a457-e46ee1d7cd4a