Reduce code duplication.

Reduce code duplication.

The (hp::)DoFHandler::n_boundary_dofs() functions were specialized for
the 1d case back in the day when faces in 1d were not usable the same
way faces in 2d/3d were accessible via iterators. Because C++ doesn't
allow partial specialization of member functions, the code was also
replicated three times for the <1,1>, <1,2>, and <1,3> cases.

Fix this: We can now always use the generic approach since the necessary
iterators were added several years ago. This also fixes a bug: in <1,2>
and <1,3>, we allow triangulations to have more than 2 end points,
which the existing code was not equipped to deal with.