Remove use of DIN_LOC_ROOT_CLMFORC

[ekluzek]

We want to remove DIN_LOC_ROOT_CLMFORC

Parent issue is:

ESMCI/cime#3097

[ekluzek]

This is defined for each machine in config_machines.xml. We can remove it here once's it's removed in CDEPS. This will make it easier for users adding a new machine as they won't have to wonder what they need to do with this variable.

Users might run out of space with the forcing data for their machines, but that becomes a problem they'll need to solve using softlinks to other disks of buying bigger disks. Somewhere we'll need to document that problem. That probably should go into the CLM documentation.