Remove getter for articulationStageKeys, it's not useful by itself.
Remove getter for articulationStageKeys, it's not useful by itself.

josh-bernstein/cesium,YonatanKra/cesium,CesiumGS/cesium,CesiumGS/cesium,YonatanKra/cesium,progsung/cesium,jasonbeverage/cesium,YonatanKra/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium,jasonbeverage/cesium,josh-bernstein/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,likangning93/cesium,YonatanKra/cesium,CesiumGS/cesium,likangning93/cesium,progsung/cesium,likangning93/cesium