Remove unused std::_Iter_base class template

This class template has been unused since __gnu_debug::__base was
removed in r263786.

	* include/bits/stl_iterator_base_types.h (_Iter_base): Remove unused
	class template and partial specialization.

From-SVN: r270647

libstdc++-v3/ChangeLog

 2019-04-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/stl_iterator_base_types.h (_Iter_base): Remove unused
+	class template and partial specialization.
+
 	PR libstdc++/87982
 	* include/bits/stl_algo.h (generate_n): Convert _Size parameter to
 	an integral type.

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

@@ -207,26 +207,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   //@}
 
-#if __cplusplus < 201103L
-  // If _Iterator has a base returns it otherwise _Iterator is returned
-  // untouched
-  template<typename _Iterator, bool _HasBase>
-    struct _Iter_base
-    {
-      typedef _Iterator iterator_type;
-      static iterator_type _S_base(_Iterator __it)
-      { return __it; }
-    };
-
-  template<typename _Iterator>
-    struct _Iter_base<_Iterator, true>
-    {
-      typedef typename _Iterator::iterator_type iterator_type;
-      static iterator_type _S_base(_Iterator __it)
-      { return __it.base(); }
-    };
-#endif
-
 #if __cplusplus >= 201103L
   template<typename _InIter>
     using _RequireInputIter = typename