PixelGeoModel: comment unused flexDensity variable to avoid compiler warning

The GeoPixelDetailedStaveSupport code used to rely on a flexDensity variable, but the code that used that variable has been commented out for now. The source comments imply that we are keeping it in case it needs to be used in the future. While the functional code is commented out, we have compiler warnings about unused variable flexDensity. This MR simply comments out that declaration to avoid the warnings.