Remove iOS 7 UICollectionView delegate API support
Remove iOS 7 UICollectionView delegate API support

Summary: Don't need support for iOS 7 anymore, save some cycles on layout.

Reviewed By: ocrickard

Differential Revision: D4098229

fbshipit-source-id: eeaaf875acda1229f9622f11acc7c77ccad75cf0

Instagram/IGListKit,charles0126dev/IGListKit,charles0126dev/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,dshahidehpour/IGListKit,Instagram/IGListKit,Instagram/IGListKit,dshahidehpour/IGListKit,Sherlouk/IGListKit,zhubofei/IGListKit,dshahidehpour/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit,Instagram/IGListKit