make BIM not float in sandcastle clipping plane demo

Author: IanLilleyT

Apps/Sandcastle/gallery/3D Tiles Clipping Planes.html

@@ -71,7 +71,6 @@
         var viewer = new Cesium.Viewer("cesiumContainer", {
           infoBox: false,
           selectionIndicator: false,
-          terrainProvider: Cesium.createWorldTerrain(),
         });
         var scene = viewer.scene;