Fix T42021: OSL doesn't work when there are non-ascii chars in the path

Quite annoying, the same thing we do from the blender side, But as a positive
side we can get rid of some utf8/utf16 conversions.

Hopefully it all work fine now, at leats works on mu russki windoze laptop.

intern/cycles/util/util_path.cpp

@@ -41,21 +41,12 @@ static string cached_user_path = "";
 
 static boost::filesystem::path to_boost(const string& path)
 {
-#ifdef _MSC_VER
-	std::wstring path_utf16 = Strutil::utf8_to_utf16(path.c_str());
-	return boost::filesystem::path(path_utf16.c_str());
-#else
 	return boost::filesystem::path(path.c_str());
-#endif
 }
 
 static string from_boost(const boost::filesystem::path& path)
 {
-#ifdef _MSC_VER
-	return Strutil::utf16_to_utf8(path.wstring().c_str());
-#else
 	return path.string().c_str();
-#endif
 }
 
 void path_init(const string& path, const string& user_path)
@@ -259,14 +250,7 @@ string path_source_replace_includes(const string& source_, const string& path)
 
 FILE *path_fopen(const string& path, const string& mode)
 {
-#ifdef _WIN32
-	std::wstring path_utf16 = Strutil::utf8_to_utf16(path);
-	std::wstring mode_utf16 = Strutil::utf8_to_utf16(mode);
-
-	return _wfopen(path_utf16.c_str(), mode_utf16.c_str());
-#else
 	return fopen(path.c_str(), mode.c_str());
-#endif
 }
 
 void path_cache_clear_except(const string& name, const set<string>& except)