Remove a 14.5 data migration

What does this MR do and why?

This removes a data migration (+ background migration and tests) introduced in 14.5 on 2021-09-21: !70872 (merged)

Per https://docs.gitlab.com/ee/update/index.html#upgrading-to-a-new-major-version, it's mandatory to upgrade to the latest minor version before going to the next major one, so it's safe to remove all data migrations that were introduced in version previous to 15.0.0, since we're currently at version 15.7.

Related to #386728 (comment 1222931385).

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.