nixos/tests: remove leftover file for removed mpich nixos test

This file is not used anymore, the test was disabled in
c044375a0a and removed in
bb8b48c794 (#19963).

nixos/tests/mpich-example.c

@@ -1,21 +0,0 @@
-#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;
-}