Drop post_migration test table

What does this MR do and why?

Drop post_migration test table.

This table was introduced as part of gitlab-com/gl-infra&585 (closed), to test the execution of the new post migration (PDM) pipeline. With the testing and rollout of the PDM being close to an end, we need to remove these test table as well.

Even though this table was introduced in 15.2, it was never referenced in the application codebase, so it's safe to remove it in this release as well.

Related to gitlab-com/gl-infra/delivery#2352 (closed)

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