Reverting an old optimization that conflicts with the new allocator model, and causes some test casees to compile that shouldn't.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@122830 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -3641,10 +3641,6 @@ swap(basic_string<_CharT, _Traits, _Allocator>& __lhs, basic_string<_CharT, _Tra
     __lhs.swap(__rhs);
 }
 
-template<class _CharT, class _Traits, class _Allocator>
-struct __is_zero_default_constructible<basic_string<_CharT, _Traits, _Allocator> >
-    : public integral_constant<bool, __is_zero_default_constructible<_Allocator>::value> {};
-
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 
 typedef basic_string<char16_t> u16string;