Clears NHR on held disable add-on action

[chrstinalin]

Fixes mozilla/addons#16041

Description

Clears NHR ('Needs Human Review') when the disable add-on enforcement action is triggered.

Testing

1. Create a high-profile add-on (i.e any promotion that's not strategic)

• Create (or upload a new version of) an add-on.
• Add the promotion via admin in Discovery Addon.
• Approve that version.

3. In reviewer tools, set NHR.
4. It should be in the manual review queue. Its NHR in admin should have is_active=true.
5. In reviewer tools, Review add-on with 'Add-on disable' enforcement action
6. Should no longer be in manual queue. Its NHR in admin should have is_active=false.
7. Should be in second-level approval queue.

For 4), Using ./manage.py fake_cinder_webhook to send a mock amo-legal-disable-addon payload should behave similarly.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.