chore(acir)!: remove UnknownWitness

[TomAFrench]

Related issue(s)

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Resolves (link to issue)

Description

Summary of changes

I've removed the functionally dead code of UnknownWitness as we don't use it anywhere.

It is in the public interface however so this is a breaking change.

Dependency additions / changes

N/A

Test additions / changes

N/A

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Additional context

(If applicable.)