sync in ncmpi_end_indep_data()

[Wei-keng Liao]

In file mpinetcdf.c, function ncmpi_end_indep_data(), MPI_File_sync() is 
called. I don't think this is necessary. Flushing dirty data may be needed 
if pnetcdf implemented a caching layer internally. However, this file sync 
should be flusing data from pnetcdf caching layer (if it is implemented) 
to the file system, not application clients to the file servers (or disks) 
as MPI_File_sync() will do.

This file sync makes IOR performance bad for pnetcdf independent data 
mode.

Wei-keng