Usually you can find answers to such questions in Google Maps API documentation.

As Andrew said, addListener returns a handle which you can use later to remove the listener. That's because a single event can have many listeners and to remove them you must save a reference to each of attached listeners.

There's also a function which removes all of the listeners at the same time:

clearListeners(instance:Object, eventName:string);

//In your case:

google.maps.event.clearListeners(map, 'bounds_changed');

Here's the Google Maps API reference where you can read about it.