Caching code seems entirely unused

[toulouse]

git grep -i cache shows mentions of caching but no actual logic.

A quick sweep shows that the following is unused:

• ASDisplayNode: NSArray *cachedNodes
• ASDisplayNode: BOOL cacheNode
• ASDisplayNode: ASDisplayNode *superCacheNode
• ASImageCacheProtocol: Unimplemented and unused

[secretiverhyme]

Thanks for the report! Some of these may be used in future by code that we haven't yet open-sourced. I've marked this as a blocker for the 1.0 release.