[libc++][NFC] Remove some dead code in common_type

[Mordante]

Nice cleanup, LGTM!

libcxx/include/__type_traits/common_type.h

struct __common_type2_imp<_Tp, _Up, __void_t<decltype(true ? std::declval<_Tp>() : std::declval<_Up>())> >
   typedef _LIBCPP_NODEBUG __decay_t<decltype(
                          true ? std::declval<_Tp>() : std::declval<_Up>()
                          )> type;
 };
 
 template <class, class = void>
 struct __common_type_impl {};
 
-// Clang provides variadic templates in C++03 as an extension.
-#if !defined(_LIBCPP_CXX03_LANG) || defined(__clang__)
-# define _LIBCPP_OPTIONAL_PACK(...) , __VA_ARGS__
 template <class... _Tp>
 struct __common_types;
 template <class... _Tp>
 struct _LIBCPP_TEMPLATE_VIS common_type;
-#else
-# define _LIBCPP_OPTIONAL_PACK(...)
-struct __no_arg;
-template <class _Tp, class _Up, class = __no_arg>
-struct __common_types;
-template <class _Tp = __no_arg, class _Up = __no_arg, class _Vp = __no_arg,
-          class _Unused = __no_arg>
-struct common_type {
-  static_assert(sizeof(_Unused) == 0,
-                "common_type accepts at most 3 arguments in C++03");
-};
-#endif // _LIBCPP_CXX03_LANG
 
 template <class _Tp, class _Up>
 struct __common_type_impl<
     __common_types<_Tp, _Up>, __void_t<typename common_type<_Tp, _Up>::type> >
 {
   typedef typename common_type<_Tp, _Up>::type type;
 };
 
-template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)>
+template <class _Tp, class _Up, class _Vp, class... _Rest>
 struct __common_type_impl<
-    __common_types<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)>,
+    __common_types<_Tp, _Up, _Vp, _Rest...>,
     __void_t<typename common_type<_Tp, _Up>::type> >
     : __common_type_impl<__common_types<typename common_type<_Tp, _Up>::type,
-                                        _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> > {
+                                        _Vp, _Rest...> > {
 };
 
 // bullet 1 - sizeof...(Tp) == 0
 
 template <>
 struct _LIBCPP_TEMPLATE_VIS common_type<> {};
 
 // bullet 2 - sizeof...(Tp) == 1
: conditional<
         _IsSame<_Tp, __decay_t<_Tp> >::value && _IsSame<_Up, __decay_t<_Up> >::value,
         __common_type2_imp<_Tp, _Up>,
         common_type<__decay_t<_Tp>, __decay_t<_Up> >
     >::type
 {};
 
 // bullet 4 - sizeof...(Tp) > 2
 
-template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)>
+template <class _Tp, class _Up, class _Vp, class... _Rest>
 struct _LIBCPP_TEMPLATE_VIS
-    common_type<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)>
+    common_type<_Tp, _Up, _Vp, _Rest...>
     : __common_type_impl<
-          __common_types<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> > {};
-
-#undef _LIBCPP_OPTIONAL_PACK
+          __common_types<_Tp, _Up, _Vp, _Rest...> > {};
 
 #if _LIBCPP_STD_VER >= 14
 template <class ..._Tp> using common_type_t = typename common_type<_Tp...>::type;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H