[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/online_file_source.cpp

@@ -177,7 +177,6 @@ OnlineFileSource::Impl::Impl(FileCache* cache_, const std::string& root)
     // loop alive; otherwise our app wouldn't terminate. After all, we only need status change
     // notifications when our app is still running.
     NetworkStatus::Subscribe(&reachability);
-    reachability.unref();
 }
 
 OnlineFileSource::Impl::~Impl() {