MappingFEField: Remove update_JxW_values for Piola transform

[bergbauer]

The comment seems outdated, see

dealii/source/fe/mapping_fe_field.cc

Lines 1979 to 1997 in b065060

	case mapping_piola:
	{
	Assert(
	data.update_each & update_contravariant_transformation,
	typename FEValuesBase<dim>::ExcAccessToUninitializedField(
	"update_contravariant_transformation"));
	Assert(
	data.update_each & update_volume_elements,
	typename FEValuesBase<dim>::ExcAccessToUninitializedField(
	"update_volume_elements"));
	Assert(rank == 1, ExcMessage("Only for rank 1"));
	for (unsigned int i = 0; i < output.size(); ++i)
	{
	output[i] =
	apply_transformation(data.contravariant[i], input[i]);
	output[i] /= data.volume_elements[i];
	}
	return;
	}

where update_volume_elements is used and the contravariant transformation

dealii/source/fe/mapping_fe_field.cc

Lines 736 to 764 in 2896a7e

	if (update_flags & update_contravariant_transformation)
	{
	const unsigned int n_q_points = data.contravariant.size();
	Assert(data.n_shape_functions > 0, ExcInternalError());
	for (unsigned int point = 0; point < n_q_points; ++point)
	{
	const Tensor<1, dim> *data_derv =
	&data.derivative(point + data_set, 0);
	Tensor<1, dim> result[spacedim];
	for (unsigned int k = 0; k < data.n_shape_functions; ++k)
	{
	const unsigned int comp_k =
	fe.system_to_component_index(k).first;
	if (fe_mask[comp_k])
	result[fe_to_real[comp_k]] +=
	data.local_dof_values[k] * data_derv[k];
	}
	// write result into contravariant data
	for (unsigned int i = 0; i < spacedim; ++i)
	{
	data.contravariant[point][i] = result[i];
	}
	}
	}

We should not set update_JxW_values when not necessary. Preparation for #15281

[kronbichler]

/rebuild