Remove faulty logic from GlobeSurfaceTileProvider when layer removed.
Remove faulty logic from GlobeSurfaceTileProvider when layer removed.

The property is named `renderable`, not `isRenderable`, but it doesn't matter anyway because this code is wrong.  If the property name is corrected, then removing the base layer produces a missing globe instead of the blue globe that you get when you don't add a base layer at construction time.

aelatgt/cesium,geoscan/cesium,esraerik/cesium,omh1280/cesium,emackey/cesium,oterral/cesium,AnalyticalGraphicsInc/cesium,geoscan/cesium,progsung/cesium,kiselev-dv/cesium,likangning93/cesium,wallw-bits/cesium,soceur/cesium,CesiumGS/cesium,emackey/cesium,aelatgt/cesium,jason-crow/cesium,esraerik/cesium,ggetz/cesium,jason-crow/cesium,kiselev-dv/cesium,denverpierce/cesium,AnimatedRNG/cesium,omh1280/cesium,AnalyticalGraphicsInc/cesium,YonatanKra/cesium,likangning93/cesium,jasonbeverage/cesium,esraerik/cesium,YonatanKra/cesium,ggetz/cesium,AnimatedRNG/cesium,progsung/cesium,wallw-bits/cesium,hodbauer/cesium,CesiumGS/cesium,YonatanKra/cesium,denverpierce/cesium,denverpierce/cesium,aelatgt/cesium,josh-bernstein/cesium,NaderCHASER/cesium,likangning93/cesium,CesiumGS/cesium,AnimatedRNG/cesium,YonatanKra/cesium,ggetz/cesium,oterral/cesium,denverpierce/cesium,NaderCHASER/cesium,jasonbeverage/cesium,soceur/cesium,likangning93/cesium,jason-crow/cesium,CesiumGS/cesium,hodbauer/cesium,omh1280/cesium,CesiumGS/cesium,omh1280/cesium,oterral/cesium,kaktus40/cesium,emackey/cesium,kiselev-dv/cesium,esraerik/cesium,ggetz/cesium,AnimatedRNG/cesium,NaderCHASER/cesium,wallw-bits/cesium,hodbauer/cesium,emackey/cesium,jason-crow/cesium,aelatgt/cesium,josh-bernstein/cesium,kaktus40/cesium,likangning93/cesium,kiselev-dv/cesium,soceur/cesium,wallw-bits/cesium