Removed bulk density from the soil model as it isn't actually used

[jacobcook1995]

Description

This PR removes all uses of bulk density from the soil model associated files as it isn't actually used there (@vgro uses it in a different way in the abiotic/hydrology models)

Fixes #1002

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

[vgro]

LGTM

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (8e4416d) to head (a0cd133).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1005      +/-   ##
===========================================
+ Coverage    94.91%   94.93%   +0.01%     
===========================================
  Files           80       80              
  Lines         6632     6631       -1     
===========================================
  Hits          6295     6295              
+ Misses         337      336       -1     

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