libstdc++: Remove redundant macro that is always empty

The __iter_swap class template and explicit specialization are only
declared (and used) for C++03 so _GLIBCXX20_CONSTEXPR does nothing here.

	* include/bits/stl_algobase.h (__iter_swap, __iter_swap<true>): Remove
	redundant _GLIBCXX20_CONSTEXPR.

libstdc++-v3/ChangeLog

+2020-02-06  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/stl_algobase.h (__iter_swap, __iter_swap<true>): Remove
+	redundant _GLIBCXX20_CONSTEXPR.
+
 2020-02-05  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/iterator_concepts.h (iter_reference_t)

libstdc++-v3/include/bits/stl_algobase.h

@@ -139,7 +139,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct __iter_swap
     {
       template<typename _ForwardIterator1, typename _ForwardIterator2>
-	_GLIBCXX20_CONSTEXPR
 	static void
 	iter_swap(_ForwardIterator1 __a, _ForwardIterator2 __b)
 	{
@@ -155,14 +154,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct __iter_swap<true>
     {
       template<typename _ForwardIterator1, typename _ForwardIterator2>
-	_GLIBCXX20_CONSTEXPR
 	static void
 	iter_swap(_ForwardIterator1 __a, _ForwardIterator2 __b)
 	{
 	  swap(*__a, *__b);
 	}
     };
-#endif
+#endif // C++03
 
   /**
    *  @brief Swaps the contents of two iterators.
@@ -205,6 +203,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	&& __are_same<_ValueType2&, _ReferenceType2>::__value>::
 	iter_swap(__a, __b);
 #else
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 187. iter_swap underspecified
       swap(*__a, *__b);
 #endif
     }