rm MAD scanner

[eviljeff]

Fixes: mozilla/addons#15849

Description

Removes all MAD scanner integration in addons-server

Context

There are three database columns (two already null-able; one with a migration to make it null-able) that will need removing in a subsequent push.

Testing

Shouldn't be necessary, but you could:

• upload an xpi so scanners run (now without mad)
• access reviewer tools (mad review queue is gone)
• load a review page and perform an action that would have clear the mad flag (e.g. approve a version)

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.

[willdurand]

There are three database columns (two already null-able; one with a migration to make it null-able) that will need removing in a subsequent push.

Do we already have a ticket for that?

[eviljeff]

There are three database columns (two already null-able; one with a migration to make it null-able) that will need removing in a subsequent push.

Do we already have a ticket for that?

mozilla/addons#15887

[willdurand]