Refactor fates user/test mods and compsets.

[mvdebolskiy]

Description of changes

Make build-namelist pick-up everything from component modifier for different fates modes, instead of relying on manually setting flags or checking out usermods.

Specific notes

**Contributors other than yourself, if any:

• (Replace this text and add more list items as needed)

CTSM issues resolved or otherwise addressed, if any:

• Resolves Remove FatesSetupParamBuild #3989

If answers are expected to change, describe (delete this line otherwise):

None.

Any user interface changes (namelist or namelist defaults changes)?

Fates modes are set through compset/CLM_BLDNML_OPTS and bgc option, so a usermod is not needed.

Testing planned or performed, if any:

• aux_clm
• fates

Requirements before merge:

• I have followed the CTSM contribution guidelines.

• The code in this PR branch builds with no errors.
• The code in this PR branch runs with no errors. Briefly describe tested configuration(s):
• This either (a) does not change answers, (b) it only changes answers at roundoff level, or (c) I have performed a scientific evaluation of the answer changes. Which?:

• I have reviewed relevant parts of the CLM documentation Tech Note or User's Guide to determine if anything needs to be changed or added. If it does, describe:
• This PR either (a) does not create a need to update the documentation or (b) includes required documentation updates (see guidelines for contributing documentation). Which?: