string: remove deprecated wstring to_uft8 convertors

The `codecvt` routines are deprecated, and this code is entirely
untested.

string.cpp

@@ -9,6 +9,9 @@
 #include "string.hpp"
 
 #include <cstring>
+
+
+#if 0
 #include <codecvt>
 #include <locale>
 
@@ -34,6 +37,7 @@ cruft::to_utf8 (const std::wstring &src)
 {
     return to_utf8 (src.c_str ());
 }
+#endif
 
 
 ///////////////////////////////////////////////////////////////////////////////