Remove unit_assignment form

[gigxz]

Merging this PR

• use the squash-merge strategy for PRs targeting a release-X branch
• use a merge-commit or rebase strategy for PRs targeting the stable branch

Description

Remove this form, which we plan to manage in the form builder instead of version control. This will allow us to set editor_user_ids on the form.

Issue: https://github.com/open-path/Green-River/issues/7093

When deployed, we'll need to:

• manually mark the FormDefinition as managed_in_version_control: false. (future improvement would be for the seed script to do that on its own)
• in the Form Builder, create and publish a new version that has editor_user_ids specified - per https://github.com/open-path/Green-River/issues/7093#issuecomment-2564830446

Type of change

config

Checklist before requesting review

• I have performed a self-review of my code
• I have run the code that is being changed under ideal conditions, and it doesn't fail
• If adding a new endpoint / exposing data in a new way, I have:
  • ensured the API can't leak data from other data sources
  • ensured this does not introduce N+1s
  • ensured permissions and visibility checks are performed in the right places
• Any major architectural changes are supported by an approved ADR (Architectural Decision Record)
• I have updated the documentation (or not applicable)
• I have added spec tests (or not applicable)
• I have provided testing instructions in this PR or the related issue (or not applicable)

//: # NOTE: system tests may fail if there is no branch on the hmis-frontend that matches the Source or Target branch of this PR. This is expected