[core] libuv unref() not really needed

The run loop will be kept alive because it has an `AsyncTask`. We
also can simple stop the loop with `uv_stop()`.

The `RunLoop` is still gonna be the last object to be destroyed
because it is the first object to be created in the thread, so
by design it won't outlive `Timer`s and `AsyncTask`s.

This patch won't change the current behavior, will just make
the code simpler.

platform/default/timer.cpp

@@ -22,7 +22,6 @@ public:
         }
 
         handle()->data = this;
-        uv_unref(handle());
     }
 
     ~Impl() {