Drops custom utf16 string implementation (from cdd3317ded), uses C++11 one.

Patch By: sera
Differential Revision: https://code.wildfiregames.com/D4223
This was SVN commit r26023.

source/scriptinterface/ScriptInterface.cpp

@@ -29,7 +29,6 @@
 #include "ps/CLogger.h"
 #include "ps/Filesystem.h"
 #include "ps/Profile.h"
-#include "ps/utf16string.h"
 
 #include <map>
 #include <string>