Retire the `pft_definition` option in the PlantsModel schema

[davidorme]

We now have the much clearer option to configure plant functional types in a single CSV file, rather than a clunky and long-winded array of arrays in TOML. We should only retain and then document one input route and the CSV is far superior so:

• Remove that from the model config
• Update tests to use some shared CSV trait files

Note that the set of variables in the schema are currently the only "documentation" of the required fields, so #1037 needs to happen first!