Drop ONLY clause for MPI Fortran module.

The ONLY clause works only with MPI versions that have all MPI subroutines
declared in the module. According to Thomas Jahns, this is not always the case
(see comments in #4 for more details).

examples/pio/collectData2003.F90

@@ -3,8 +3,7 @@ PROGRAM collectdata2003
   USE yaxt, ONLY: xt_initialize, xt_finalize, xt_idxlist, xt_idxstripes_new, &
        xt_idxlist_delete, xi => xt_int_kind, xt_stripe
   USE iso_c_binding, ONLY: c_int
-  USE mpi, ONLY: mpi_init, mpi_comm_rank, mpi_comm_size, mpi_barrier, &
-       mpi_finalize, mpi_comm_world, mpi_comm_null, mpi_success
+  USE mpi
 #endif
 
   IMPLICIT NONE