Remove Kml converter

The converter is out of date and unmaintained and Cesium's in-development support has superseded it.

My current plan is to have a `kml` "release" branch in Cesium which tracks the latest official build.  Another options would be to make KML a plug-in for the time being; but I'd like to think we'll have it in `master` within a few months.
I'll just remove this and merge.