Remove usage of addhandler_callback

[piever]

Observables' technique of using two global lists of callbacks

const addhandler_callbacks = []
const removehandler_callbacks = []

which packages would modify on load doesn't seem ideal and IMO we could think about removing it.

Is this something that WebIO needs, or can it be removed? From what I understand, only this handler is registered, but it doesn't seem to do anything (unless I'm missing something of course).