Deprecate and remove the CLMCRUv7 forcing option

In the CTSM SE meeting we decided we could deprecate the remove the CLMCRUv7 forcing option that was used for clm4_5 which is also being removed.

Definition of done:
- Remove uses of CRUv7 in datm/cime_config
- Remove instances of CLMCRU in datm/cime_config and doc/source/datm.rst
- Make sure testlist runs and gives identical results with the change (just drops a test)

Start by searching datm/cime_config for CRUv7 and CLMCRU, then inspect the related entries in doc/source/datm.rst. Run the testlist before and after the change; done means the forcing references are removed and the testlist produces identical results apart from the dropped test.

Written by the indexing model from the issue text.