Application: remove a default value.

This'll stop Clang-Tidy's complaining that "default values in
virtual/override methods are prohibited". Dunno how prohibited they
are, but better safe than sorry, even though there was no compile
error/warning.

src/Application/Application.h

@@ -63,7 +63,7 @@ class Application: public Platform::Sdl2Application, public efsw::FileWatchListe
                               const std::string& dir,
                               const std::string& filename,
                               efsw::Action action,
-                              std::string old_filename = "") override;
+                              std::string old_filename) override;
 
     private:
         // Events