Remove config_cache.xml used by CLMBuildNamelist

With the removal of CLM4 (#455 ), the config_cache.xml file used in CLMBuildNamelist isn't serving much purpose any more: I think it now just contains the phys version. It seems like we should remove the use of this file and just pass the phys option to CLMBuildNamelist as a command-line flag. I started briefly down this path, but got lost.

@ekluzek I'm thinking (hoping) that you understand the relevant infrastructure better than I do, so maybe this is something you could either do quickly yourself or at least give me some tips when we're both back in the office. No rush on this, though - I just wanted to record this as a possible cleanup we could do at some point.