Remove "Attribute name for net ID" from subject source

This is a field that shows up in subject source configuration, but there is no explanation what a "net ID" is for. I had traced through the source code a while ago, and at that time it was only used in either the custom UI or a less common reporting setup. It would eliminate some confusion if it were just refactored to not use this custom net ID. It we do need some kind of preferred identifier, then maybe it just needs to be rebranded – renaming and adding a description.