[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();
 }
 

src/mbgl/util/async_task.hpp

@@ -15,7 +15,6 @@ public:
     ~AsyncTask();
 
     void send();
-    void unref();
 
 private:
     class Impl;

src/mbgl/util/timer.hpp

@@ -17,7 +17,6 @@ public:
 
     void start(Duration timeout, Duration repeat, std::function<void()>&&);
     void stop();
-    void unref();
 
 private:
     class Impl;