AcceleratorController has a handful of functions that don't make sense once we've migrated to mus. Remove them. Specifically:

  void Register(const ui::Accelerator& accelerator,                              |
                ui::AcceleratorTarget* target);                                  |
  void Unregister(const ui::Accelerator& accelerator,                            |
                  ui::AcceleratorTarget* target);                                |
  void UnregisterAll(ui::AcceleratorTarget* target);