Retire the plant schema [pft_definition] option

[davidorme]

Description

This PR simplifies the config options for the PlantsModel by removing the TOML array [[plants.pft_definition]] approach completely and only using the pft_definitions_path option.

Fixes #1038

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

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.20%. Comparing base (975a39a) to head (1d18d26).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...irtual_ecosystem/models/plants/functional_types.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1060      +/-   ##
===========================================
- Coverage    95.25%   95.20%   -0.05%     
===========================================
  Files           81       81              
  Lines         6895     6889       -6     
===========================================
- Hits          6568     6559       -9     
- Misses         327      330       +3     

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[sallymatson]

LGTM!