string_util: remove TString conversion for windows

First of all they are foundamentally broken. As our convention is that std::string is always UTF-8, these functions assume that the multi-byte character version of TString (std::string) from windows is also in UTF-8, which is almost always wrong. We are not going to build multi-byte character build, and even if we do, this dirty work should be handled by frontend framework early.

src/common/file_util.h

@@ -285,7 +285,7 @@ private:
 template <typename T>
 void OpenFStream(T& fstream, const std::string& filename, std::ios_base::openmode openmode) {
 #ifdef _MSC_VER
-    fstream.open(Common::UTF8ToTStr(filename).c_str(), openmode);
+    fstream.open(Common::UTF8ToUTF16W(filename).c_str(), openmode);
 #else
     fstream.open(filename.c_str(), openmode);
 #endif