drop support for AMO_ESCALATE and forwarded jobs

[eviljeff]

Fixes: mozilla/addons#15542

Description

Deletes a big chunk of now unused code

This PR should not be merged until https://sql.telemetry.mozilla.org/queries/107270 is empty, or we are confident will be empty before the deploy to production.

Context

Legal escalations still escalate by creating a new job (because Cinder doesn't allow queue moves via the API currently). But we only needed all the foreign keys and NHRs when the job was shown in the reviewer tools., so it significantly simplifies the code.

Testing

No specific testing is needed as it's removing code, but you could re-test the legal escalation functionality, as there is a slight refactoring around that, and there was some shared code .

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.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[eviljeff]

merge blocked on mozilla/addons#15574 (if we do, then when those jobs are closed the associated NHR won't be closed)