Surface_mesh: remove obsolete g++4.4 workaround from SM_Halfedge_index

CGAL 6.0 raised the minimum compiler requirement to GCC 11.4 (and
equivalently for Clang/MSVC).  The work around that was added to
SM_Halfedge_index to paper over a g++ 4.4 ADL bug for 'std::next'
lookups is therefore no longer needed.

The workaround added five 'typedef void' members
(iterator_category, value_type, difference_type, pointer, reference)
to SM_Halfedge_index so that it superficially looked like a legacy
input-iterator.  This tricked g++ 4.4's ADL machinery into skipping
the std::next overload set when an SM_Halfedge_index was an argument,
but it has the unwanted side-effect of satisfying the legacy
iterator_traits check inside pybind11.  Specifically,
pybind11::detail::is_move_constructible<SM_Halfedge_index> evaluates
as false because pybind11 inspects value_type and, finding 'void',
concludes the type is not a proper value type and therefore not move-
constructible -- contradicting std::move_constructible<SM_Halfedge_index>
which correctly evaluates as true.

Fix: remove the five typedef void members and the comment explaining
them.  No other code in the CGAL codebase depends on these typedefs.

Fixes #9319

Author: RajdeepKushwaha5

Surface_mesh/include/CGAL/Surface_mesh/Surface_mesh.h

@@ -162,15 +162,6 @@ namespace CGAL {
     {
     public:
 
-        // Workaround for a bug in g++4.4 in ADL for function next:
-        // we provide the types needed for std::iterator_traits<Surface_mesh::halfedge_index>,
-        // although this descriptor is not an iterator.
-        typedef void iterator_category;
-        typedef void value_type;
-        typedef void difference_type;
-        typedef void pointer;
-        typedef void reference;
-
         SM_Halfedge_index() : SM_Index<SM_Halfedge_index>((std::numeric_limits<size_type>::max)()) {}
 
         explicit SM_Halfedge_index(size_type _idx) : SM_Index<SM_Halfedge_index>(_idx) {}