Refactor and remove the use_case files

Besides the complexity that use_case options provide as recognized in #1763, we frequently get confused as to what should go in the use_case files -- and what goes elsewhere (for example #3621). Also as seen in #3621 there are times when you can't add the needed attributes to a use_case -- because of order dependence problems (see this comment: https://github.com/ESCOMP/CTSM/issues/3621#issuecomment-3568926968).

A good way to get away from this would be to have build-namelist remove the files in the directory -- and make sure their content is put in namelist_defaults_ctsm.xml. But, add a new namelist attribute that can be used in the namelist_defaults_ctsm.xml for use_case that comes from the CLM_NML_USE_CASE XML item. So it does what #1763 asks for in moving things to XML variables, just for an XML variable that already exists.