Remove prefix additions

Cocoapods doesn't wrap the prefix additions in `#ifdef __OBJC_`. This causes a problem when using a pod like `BotKit` alongside a pod like `SSZipArchive`, which includes non-objective-c files. This is a problem with cocoapods, but in all actuality, we don't really need to be adding `CoreData` and `Availability` to the prefix in the first place. So it's probably better to just move these imports to the files that need them.