Kernel: Remove ancient unused Scheduler::beep() declaration

Kernel/Scheduler.h

@@ -64,7 +64,6 @@ public:
     static void prepare_after_exec();
     static void prepare_for_idle_loop();
     static Process* colonel();
-    static void beep();
     static void idle_loop(void*);
     static void invoke_async();
     static void notify_finalizer();