Remove terraform-docs workflow

Nice idea, but GHA can't push to protected branches:

* https://github.com/orgs/community/discussions/25305

See also:

* 🔒 https://trello.com/c/wMX33z55/178-investigate-options-for-gha-to-apply-terraform-docs-changes-to-open-prs-that-change-formatting

Author: rpdelaney