Remove/replace refs to DATM_C*_YR_* variables

[slevis-lmwg]

Description of changes

As in title.

Specific notes

Contributors other than yourself, if any:
@samsrabin

CTSM Issues Fixed (include github issue #):
Fixes #2743

Are answers expected to change (and if so in what way)?
No

Does this create a need to change or add documentation? Did you do so?
Yes

[slevis-lmwg]

Thank you for approving, @samsrabin.

What's your opinion regarding testing here:

• I'm tempted to merge without testing (though buildnml is not a documentation file).
• So should I run build-namelist_test.pl when derecho comes back?
• Or should I just run all testing and not overthink it?

[samsrabin]

Thank you for approving, @samsrabin.

What's your opinion regarding testing here:

• I'm tempted to merge without testing (though buildnml is not a documentation file).
• So should I run build-namelist_test.pl when derecho comes back?
• Or should I just run all testing and not overthink it?

I think running all NEON tests and then the clm-short suite, plus the build namelist test, should be sufficient.

[samsrabin]

OH, sorry, I thought this was #2855. I think the build-namelist test only should be fine.

[slevis-lmwg]

PASS ./build-namelist_test.pl
FAIL python -s testing BUT error is related to yesterday's derecho updates and occurs in b4b-dev before merging this PR

[slevis-lmwg]

UPDATE
PASS python -s testing after manually running "module load cdo"