Removed positionMC from czm_materialInput.

It is no longer used by any materials and is not potentially useful enough to material authors to keep it.

Surface shaders will, of course, allow for custom material inputs, and may happen sooner than I thought now that I am looking more carefully at batching.

Also updated the [Fabric wiki](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Fabric).