Calling the correct handler removal function for the disposable

The disposable attempts to call the #off() method on the event emitter, crashing node.js. Node.js doesn't have an #off() method on it's EventEmitter class, it only has a #removeListener() function.

http://nodejs.org/api/events.html#events_emitter_removelistener_event_listener
https://github.com/joyent/node/blob/master/lib/events.js