[core] Get rid of ::unref() for Timer and AsyncTask

Not need, legacy from libuv. The RunLoop keep the main
loop running until is explicitly no longer needed.

platform/default/async_task.cpp

@@ -28,6 +28,7 @@ public:
         }
 
         handle()->data = this;
+        uv_unref(handle());
     }
 
     ~Impl() {
@@ -43,10 +44,6 @@ public:
         }
     }
 
-    void unref() {
-        uv_unref(handle());
-    }
-
 private:
     static void asyncCallback(UV_ASYNC_PARAMS(handle)) {
         reinterpret_cast<Impl*>(handle->data)->task();
@@ -71,9 +68,5 @@ void AsyncTask::send() {
     impl->maySend();
 }
 
-void AsyncTask::unref() {
-    impl->unref();
-}
-
 } // namespace util
 } // namespace mbgl