Remove depreacted pre_install/post_install hooks

These are finally gone.

See http://blog.cocoapods.org/CocoaPods-Trunk/
Re https://github.com/CocoaPods/trunk.cocoapods.org/issues/33
Closes https://github.com/CocoaPods/trunk.cocoapods.org/issues/55
Merge pull request #12052 from CocoaPods/trunk

Remove depreacted pre_install/post_install hooks