[core] Remove RunLoop::{ref,unref}

include/mbgl/util/run_loop.hpp

@@ -41,9 +41,6 @@ public:
     void runOnce();
     void stop();
 
-    void ref();
-    void unref();
-
     // So far only needed by the libcurl backend.
     void addWatch(int fd, Event, std::function<void(int, Event)>&& callback);
     void removeWatch(int fd);