remove old background migrations

What does this MR do and why?

In this MR, we are removing old background migrations.

This is a follow-up MR. We already removed the regular and post migrations that used those background migrations. !76921 (merged)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Please take a look here

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #331248 (closed)