Servo dispatches too many disconnected callbacks.

[emilio]

I found this while reviewing #23208, though it's pre-existing and probably dates back to the initial custom-elements implementation.

ScriptThread::enqueue_callback_reaction is called with CallbackReaction::Disconnected every time a node is unbound from a tree, but that is not correct. That bit should be conditional on the node being connected before, since otherwise you get callbacks for stuff like removing a custom element in a already-unbound subtree.

cc @cbrewster @jdm

[emilio]

Filed whatwg/dom#773 since the spec is wrong on this one I believe.