Clean up one more DoFHandlerPolicy thing.

Clean up one more DoFHandlerPolicy thing.

We had a function with a funny comment suggesting that a compiler did not
correctly understand the code. But the comment is wrong: when the only
argument to a function that references a 'dim' or 'spacedim' template
argument is of the form
  onst typename DoFHandler<dim,spacedim>::level_cell_iterator &
then 'dim' and 'spacedim' are simply not deducible. That's how C++ works.

The function therefore had a dummy argument of type DoFHandler<dim,spacedim>
that isn't used. That's awkward, but works. But we have a way to deal
with the lack of deducibility that's used elsewhere in the library
namely internal::int2type. Use this approach here as well, and remove
the misleading comment.