nixos/tests: remove leftover file for removed mpich nixos test (#451288)

#include <stdio.h>

#include <stdlib.h>

#include <mpi.h>

int

main (int argc, char *argv[])

{

  int rank, size, length;

  char name[BUFSIZ];

  MPI_Init (&argc, &argv);

  MPI_Comm_rank (MPI_COMM_WORLD, &rank);

  MPI_Comm_size (MPI_COMM_WORLD, &size);

  MPI_Get_processor_name (name, &length);

  printf ("%s: hello world from process %d of %d\n", name, rank, size);

  MPI_Finalize ();

  return EXIT_SUCCESS;

}