Do not build PnetCDF.

Use Open MPI only for parallel support instead of both PnetCDF and Open MPI. Related pnetcdf-openmpi#1.

Current configuration summary limited to Parallel NetCDF:

Building Parallel NetCDF
-- Using PnetCDF:       ON
-- Using Parallel IO:   ON

After change:

Building Parallel NetCDF
-- Using PnetCDF:       OFF
-- Using Parallel IO:   ON