[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/timer.cpp

@@ -22,6 +22,7 @@ public:
         }
 
         handle()->data = this;
+        uv_unref(handle());
     }
 
     ~Impl() {
@@ -44,10 +45,6 @@ public:
         }
     }
 
-    void unref() {
-        uv_unref(handle());
-    }
-
 private:
     static void timerCallback(UV_TIMER_PARAMS(handle)) {
         reinterpret_cast<Impl*>(handle->data)->cb();
@@ -78,9 +75,5 @@ void Timer::stop() {
     impl->stop();
 }
 
-void Timer::unref() {
-    impl->unref();
-}
-
 } // namespace util
 } // namespace mbgl