SKYAsset.h and SKYAsset_Private.h both contain url make ref count wrong

[rickmak]

@howawong run into a nil pointer exception after called copy method on SKYAsset. the nil point is on @property (nonatomic, readwrite, copy) NSURL *url;.

To fix the mis-behaviour, we just delete the @property (nonatomic, readwrite, copy) NSURL *url; at SKYAsset_Private.h.

Our guess is the redefine of attribute in both SKYAsset.h and SKYAsset_Private.h hit a compiler bug. But we are still lack of minimal test project to demo this. I think at least @howawong can push the crashing code in some brach for diagnosis.

This issue is to identify and confirm the issue. Make sure other *_Private.h in our project don't repeat the mistake.