Remove use of DIN_LOC_ROOT_CLMFORC in CTSM, but document how to use softlinks when needed

For usability we plan to remove the DIN_LOC_ROOT_CLMFORC variable in our submodules. And we should remove it from documentation as well. But, we should give instructions on how to use softlinks in order to have DATM forcing files to be in a different disk if that becomes a data management issue.

The submodule parent issue is here:

https://github.com/ESMCI/cime/issues/3097

Definition of done:
- Complete the list of removals from CDEPS, ccs_config, and CMEPS as defined above
- Add a section to Users's Guide that talks about how to handle this if disk space becomes a problem