CI: Fix linting by removing unused variable

Co-authored-by: Semphris <semphris@protonmail.com>

Author: Semphriss

src/supertux/main.cpp

@@ -636,7 +636,6 @@ Main::launch_game(const CommandLineArguments& args)
       // we have a normal path specified at commandline, not a physfs path.
       // So we simply mount that path here...
       std::string dir = FileSystem::dirname(start_level);
-      const std::string filename = FileSystem::basename(start_level);
       const std::string fileProtocol = "file://";
       const std::string::size_type position = dir.find(fileProtocol);
       if (position != std::string::npos) {