Fix global class cache file not present when no class name

[Hilderin]

This PR fixes an issue where the global_script_class_cache.cfg could be missing on disk when the project contains no scripts.

The original draft was #94540

I added save_global_classes at 2 places:

• In EditorFileSystem::_update_script_classes when no scripts and the file does not exists on file.
• In EditorFileSystem::_remove_invalid_global_class_names when global class names are removed at startup.

Also removed ScriptServer::get_global_class_cache_file_path() which is unused.

[akien-mga]

Thanks!