Simplify declaration of DataOutStack

[bangerth]

#12322 made me look up the DataOutStack class, which has this beautiful declaration:

/**
 * @deprecated Use DataOutStack<dim, spacedim> instead.
 */
template <int dim, int spacedim = dim, typename DoFHandlerType = void>
class DataOutStack;


template <int dim, int spacedim, typename DoFHandlerType>
class DataOutStack : public DataOutStack<dim, spacedim, void>
{
public:
  DEAL_II_DEPRECATED
  DataOutStack()
    : DataOutStack<dim, spacedim, void>()
  {}
};


template <int dim, int spacedim>
class DataOutStack<dim, spacedim, void> : public DataOutInterface<dim + 1>
{
  ...

It is certainly innovative to derive X<T> from X<void> -- but also confusing :-)

I think we can remove this now.

[marcfehling]

We should agree on a point up to which we want to be backwards compatible to the old hp::DoFHandler formalism and the DoFHandlerType templates. Release 10.0 might be a bit early -- but I have no strong opinion on that.

[bangerth]

The old declaration was already deprecated, so I think it's fair to remove it.

[marcfehling]

We just deprecated the Legacy:: namespace with the transition classes in that regard.

I would prefer if we remove anything related to DoFHandlerType at once (Legacy classes and others). So maybe wait until Release 10.1...?

[bangerth]

OK.

[marcfehling]

Related to #10333.