Fix issue where the deprecated Element#childOf was not initially present on elements.

We added it to the Element.Methods object in deprecated.js _after_ the initial call to Element.addMethods, meaning that it didn't get copied over unless the user made a subsequent call to Element.addMethods for whatever reason. Unit tests didn't catch this because they call Element.addMethods before testing starts.
Remove the event registry from an element when its last observer is removed.