Remove redundant depends sections from model schema

[davidorme]

The model JSON schema contain "depends" sections that we used to use to determine model execution order. This has been superseded by #548 , so we can should remove these sections and update the docs accordingly.

The stuff to be removed in each model JSON schema is the section like this:

            "depends": {
               "type": "object",
               "default": {},
               "properties": {
                  "init": {
                     "type": "array",
                     "default": [],
                     "items": {
                        "type": "string"
                     }
                  },
                  "update": {
                     "type": "array",
                     "default": [
                        "abiotic_simple"
                     ],
                     "items": {
                        "type": "string"
                     }
                  }
               }
            },

The corresponding docs then need to be replaced with a general statement about how VE tries to resolve the model execution order based on the variables:

virtual_ecosystem/docs/source/development/design/defining_new_models.md

Lines 375 to 408 in 27e7ff6

	#### Model dependencies
	Your model may depend on a particular execution order for other models. For example, the
	`freshwater` model might rely on data set up by the `hydrology` model, and so the
	`hydrology` model needs to be initialised and updated before the `freshwater` model.
	This is controlled using model configuration: although these dependencies may be
	strong, it is more flexible to set them up through the configuration process than by
	hard coding dependencies into the model objects themselves.
	Your JSON Schema document therefore needs to include the following at the root level, so
	that the model configuration includes a `[freshwater.depends]` section:
	```json
	"depends": {
	"type": "object",
	"default": {},
	"properties": {
	"init": {
	"type": "array",
	"default": ["hydrology"],
	"items": {
	"type": "string"
	}
	},
	"update": {
	"type": "array",
	"default": ["hydrology"],
	"items": {
	"type": "string"
	}
	}
	}
	}
	```