GN: Move towards only using / on Windows

tools/gn/filesystem_utils.h

Index: tools/gn/filesystem_utils.h
diff --git a/tools/gn/filesystem_utils.h b/tools/gn/filesystem_utils.h
index f720987050f27ab26acbb206a6b70d75ccf42dad..b7cec49773202306b8947b5273dbcc12ba155006 100644
--- a/tools/gn/filesystem_utils.h
+++ b/tools/gn/filesystem_utils.h
@@ -135,11 +135,6 @@ std::string InvertDir(const SourceDir& dir);
 // Collapses "." and sequential "/"s and evaluates "..".
 void NormalizePath(std::string* path);
 
-// Converts slashes to backslashes for Windows. Keeps the string unchanged
-// for other systems.
-void ConvertPathToSystem(std::string* path);
-std::string PathToSystem(const std::string& path);
-
 // Takes a source-absolute path (must begin with "//") and makes it relative
 // to the given directory, which also must be source-absolute.
 std::string RebaseSourceAbsolutePath(const std::string& input,