Remove dead code related to compositing from Sky

sky/engine/public/platform/Platform.h

Index: sky/engine/public/platform/Platform.h
diff --git a/sky/engine/public/platform/Platform.h b/sky/engine/public/platform/Platform.h
index e9a8ee8ccdb577fe2121e978b69f24623c613f3e..8923902ad604e17b7f0cb990aed2f5a04ea3c321 100644
--- a/sky/engine/public/platform/Platform.h
+++ b/sky/engine/public/platform/Platform.h
@@ -345,8 +345,6 @@ public:
     // This value must be checked again after a context loss event as the platform's capabilities may have changed.
     virtual bool canAccelerate2dCanvas() { return false; }
 
-    virtual bool isThreadedCompositingEnabled() { return false; }
-
 protected:
     virtual ~Platform() { }
 };