mpi/src/starpu_mpi_collective.c: fix code for starpu_mpi_scatter_detached (at least i hope, old code not commented is tricky to understand)

mpi/src/starpu_mpi_collective.c

@@ -57,7 +57,6 @@ int starpu_mpi_scatter_detached(starpu_data_handle_t *data_handles, int count, i
 		callback_arg->nb = 0;
 		callback_arg->callback = (rank == root) ? scallback : rcallback;
 		callback_arg->arg = (rank == root) ? sarg : rarg;
-		if (callback_arg->callback == NULL)
 
 		for(x = 0; x < count ; x++)
 		{