Delete unused background migration code

What does this MR do?

👷 Professional code deletion in progress!

This MR is a follow-up to !31936 (merged) which cleaned up old database migrations (introduced in 2019). We skipped over deleting the referenced background migration code back then - this is what this change does.

I basically went through all post-migration from !31936 (merged), looked for background migration classes and performed a quick code search for the class name. If nothing referenced those classes anymore, I deleted the file and the spec for it.

We might well still have unreferenced background migrations in the codebase, so I haven't checked if this is complete. It's a start though and should target background migrations from everything that was cleaned up with !31936 (merged).