MotorController: no need to make this function a class function

The `StopRinging()` function was used just in `Notifications.h` screen.
That screen already has access to a `motorController` object.

src/components/motor/MotorController.h

@@ -12,7 +12,7 @@ namespace Pinetime {
       void Init();
       void RunForDuration(uint8_t motorDuration);
       void StartRinging();
-      static void StopRinging();
+      void StopRinging();
 
     private:
       static void Ring(void* p_context);