clean up the code for has3DTilesExtensionSpec
clean up the code for has3DTilesExtensionSpec

CesiumGS/cesium,likangning93/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,likangning93/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium