Fix mobile app submodule initialization and cleanup

- Removed incorrect swingmusic_mobile directory
- Properly initialized swingmusic_mobile submodule from swingmusic-mobile.git
- Clean submodule configuration for unified release workflow

swingmusic_mobile/linux/runner/my_application.h

@@ -1,21 +0,0 @@
-#ifndef FLUTTER_MY_APPLICATION_H_
-#define FLUTTER_MY_APPLICATION_H_
-
-#include <gtk/gtk.h>
-
-G_DECLARE_FINAL_TYPE(MyApplication,
-                     my_application,
-                     MY,
-                     APPLICATION,
-                     GtkApplication)
-
-/**
- * my_application_new:
- *
- * Creates a new Flutter-based application.
- *
- * Returns: a new #MyApplication.
- */
-MyApplication* my_application_new();
-
-#endif  // FLUTTER_MY_APPLICATION_H_