AO3-7590 Remove unused required-field.html help file

[aaaaaasi]

Pull Request Checklist

• Have you read the contributing guidelines?
• Have you read "How to write the perfect pull request"?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.
• I acknowledge that submitting code created or modified with the help of AI is a bannable offense.

Issue

https://otwarchive.atlassian.net/browse/AO3-7590

Purpose

Remove unused required-field.html help file.

Testing Instructions

No testing.

Credit

UWNE

[Bilka2]

Thank you!