Remove subsystems core_register and core_publish

Both of them do not have any autoloaded classes, no language string files,
both of them are located inside other subsystems ('admin' and 'course' respectively). I don't see a reason why we need these subsystems. Subsystem 'core_hub' is responsible for storing the language strings and autoloaded classes for both site registration and course publishing.

This issue was raised because if these subsystems exist we would need to implement Privacy API for them.