Kill it.

The `\OC\Hook\Emitter` interface should be made available in public if it isn't yet.

I would say that this is the only good moment there is to drop the old hooks system, the main use cases for hooks (files, users) are using the new emitter system now and anything still relying on the old system should be wrapped by the new system when it's made public.

If we don't kill the old hooks system now we will never be able to kill it.