[WIP] Remove old categories in favor of the new taxonomy field

[jwass]

Major change release plan

This is a major breaking change as it removes a field.

A. Expected release date for this MAJOR change

3/20206

C. Public documentation and messaging plan

TODO.

Description

Now that we have the new taxonomy field, remove the old categories field

Reference

List of relevant links to GitHub issues, PRs, and other documentation.

1. TODO.

Testing

Test suite

Checklist

Checklist of tasks commonly-associated with schema pull requests. Please review the relevant checklists and ensure you do all the tasks that are required for the change you made.

1. Add relevant examples.
2. Add relevant counterexamples.
3. Update any counterexamples that became obsolete. For example, if a counterexample uses property A but is not intended to test property A's validity, and you made a schema change that invalidates property A in that counterexample, fix the counterexample to align it with your schema change.
4. Update in-schema documentation using plain English written in complete sentences, if an update is required.
5. Update Docusaurus documentation, if an update is required.
6. Review change with Overture technical writer to ensure any advanced documentation needs will be taken care of, unless the change is trivial and would not affect the documentation.

Documentation website

Update the hyperlink below to put the pull request number in.

[Docs preview for this PR.](https://dfhx9f55j8eg5.cloudfront.net/pr/<PUT THE PR # HERE>)

[jenningsanderson]

A major breaking change! Good to see.