No-op FixMergeRequestDiffCommitUsers background migration

What does this MR do and why?

No-op FixMergeRequestDiffCommitUsers background migration. This background migration was referring to models like Project without duplicating the logic from the model. To avoid tests failing in the future, we no-op this bg migration.

Related issue: #344540 (closed)

We can do this on a major milestone, and the background migration should have run by now, and also we cleaned up in !86320 (merged)

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