[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.

src/mbgl/map/map_context.cpp

@@ -39,9 +39,6 @@ MapContext::MapContext(View& view_, FileSource& fileSource, MapMode mode_, GLCon
     util::ThreadContext::setFileSource(&fileSource);
     util::ThreadContext::setGLObjectStore(&glObjectStore);
 
-    asyncUpdate.unref();
-    asyncInvalidate.unref();
-
     view.activate();
 }