[pulse][objc] fix NSDictionary getters
[pulse][objc] fix NSDictionary getters

Summary:
NSDIctionary models for `objectForkey` and `objectForKeyedSubscript`
were wrongly propagating the history from the key to the return value
and trying to dereference the key for no reason

Reviewed By: dulmarod

Differential Revision: D40350977

fbshipit-source-id: bd15b1bc7671eb9311b3dc81fde40037453033f0

facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer