Remove debian:binary-packages artifacts

It goes back to the earliest days of Debusine's sbuild task, where we had an artifact representing the output of sbuild (an upload) without including the .changes file of the upload itself.

This artifact was a little unwieldy, because it represented multiple binary packages. Very quickly, we added a debian:binary-package artifact to represent a single binary package, but didn't stop creating the debian:binary-packages artifacts, to minimize breakage.

However, a lot of time has passed and there's really no good reason to keep these around any longer.

Plan:

1. Stop generating the artifacts (in builds or client uploads) !2444 (merged)
2. Stop using the artifacts in tests !2445 (merged)
3. Stop accepting the artifacts in tasks !2454 (merged)
4. Wait for forky's release (or another backwards-incompatible client change that allows us to stop waiting)
5. Remove the artifacts completely, including deletion of existing artifacts from the database. !2465