remove unused renderAsync() function

include/mbgl/map/map.hpp

@@ -63,7 +63,6 @@ public:
 
     // Triggers a synchronous or asynchronous render.
     void renderSync();
-    void renderAsync();
 
     // Notifies the Map thread that the state has changed and an update might be necessary.
     void update(Update update = Update::Nothing);

src/mbgl/map/map.cpp

@@ -46,10 +46,6 @@ void Map::renderSync() {
     context->invokeSync(&MapContext::render);
 }
 
-void Map::renderAsync() {
-    context->invoke(&MapContext::render);
-}
-
 void Map::update(Update update_) {
     context->invoke(&MapContext::triggerUpdate, update_);
 }