fix: delete grants on user delete

[BruceMacD]

• when an identity is deleted remove their grants also
• test refactoring

Summary

When a user is deleted through the API we should also clean up their grants. Otherwise they hang around and cause errors when the CLI tries to look up the subject of these grants.

Checklist

• Wrote appropriate unit tests
• Considered security implications of the change
• Updated associated docs where necessary
• Updated associated configuration where necessary
• Change is backwards compatible if it needs to be (user can upgrade without manual steps?)
• Nothing sensitive logged
• Commit message conforms to Conventional Commit
• GitHub Actions are passing
• Considered data migrations for smooth upgrades

Related Issues

Resolves #1439