Remove DIN_LOC_ROOT_CLMFORC variable in favor of softlinks on machines that need it

In our CSEG meeting this morning we decided we should remove the DIN_LOC_ROOT_CLMFORC env variable, who's purpose is to put DATM forcing files on a different disk than the main DIN_LOC_ROOT directory. The current check_input_data script can't figure out this distinction so it fails when trying to download data. The data is stored in such a way to assume DIN_LOC_ROOT_CLMFORC=$DIN_LOC_ROOT/atm/datm7, and if it's different than that it won't work. Some discussion on this is here...

https://bb.cgd.ucar.edu/clm5-atmospheric-forcings-data

On machines that need it we can use softlinks. So we'll start doing that on cheyenne for example.

Definition of done:

- [ ] Do this in CDEPS ESCOMP/CDEPS#332
- [ ] Do this ccs_config_cesm ESMCI/ccs_config_cesm#226
- [ ] Remove this in ccs_config_e3sm
- [ ] Remove this in E3SM
- [ ] Do this in CMEPS ESCOMP/CMEPS#555
- [ ] Remove here in cime (this has to be last because of the XSD definition)