Fix mpi_pxlu example start

We have submitted MPI communications but not started the task graph yet,
so we cannot wait for MPI communications to terminate.

mpi/examples/mpi_lu/pxlu.c

@@ -899,8 +899,6 @@ double STARPU_PLU(plu_main)(unsigned _nblocks, int _rank, int _world_size, unsig
 		starpu_iteration_pop();
 	}
 
-	int wait_ret = starpu_mpi_wait_for_all(MPI_COMM_WORLD);
-	STARPU_ASSERT(wait_ret == MPI_SUCCESS);
 	int barrier_ret = starpu_mpi_barrier(MPI_COMM_WORLD);
 	STARPU_ASSERT(barrier_ret == MPI_SUCCESS);