Delete unused variables from soil/litter testing

[jacobcook1995]

Description

This PR deletes all the variables that are not described in the variables.toml from the places where they are used. Primarily this involves removing places where I just added extra variables to the data object in testing for convenience's sake.

@davidorme, I've removed "leaf_lignin" from the plants model entirely, as the litter model only uses "senesced_leaf_lignin", and I don't think a difference in senesced concentrations makes sense for lignin (only for N and P). Let me know if you are happy with this change, happy to take a different approach if you object to this change.

Fixes #1308

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.85%. Comparing base (70504e7) to head (894bcbd).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1358      +/-   ##
===========================================
- Coverage    94.85%   94.85%   -0.01%     
===========================================
  Files           71       71              
  Lines         7392     7388       -4     
===========================================
- Hits          7012     7008       -4     
  Misses         380      380              

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[davidorme]

Just to note that I asked @jacobcook1995 to remove test_PlantsModel_calculate_turnover_constant_override. The test was very artificial, testing model configuration but via a nasty hack to a fixture, that bizarrely turned out to propagate across tests using the fixture. It really shouldn't have, but it did.

[davidorme]

👍