Event listeners are not being removed when an entity is destroyed

[ggetz]

Adding entities (or any kind of Cesium object with a definitionChanged object) to an Entity as a non-standard key causes a listener to be added to it, and not removed.

Here's a Sandcastle example showing the issue: https://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=f0bbec7cd261365aaadd3bb991c4261c

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/bRk7kmcApeo

[ggetz]

Related to #5827