Deprecate setting of specific files with CLM_USRDAT_NAME mode

Setting CLM_USRDAT_NAME adds a useful ability to setup a case for an arbitrary resolution, without having to add it into config_grids in cime. When it was initially added it also provided a mechanism to use special names for files so that they could be used in that same case. With user_nl_clm, shell_commands and user-mod directories now -- this ability to name files this way isn't as useful anymore. I propose that it should be deprecated, and eventually removed.