Cleanup obsolete after_commit logic on User

What does this MR do and why?

Resolves #344134 (closed)

In !70872 (merged) we left some logic in after_commit hook to ensure backward compatibility. This logic should be removed once the MR is fully released, the background migration has run and the clean up after that migration is done as well (#344132 (closed)).

Note: !81916 (merged) has to be merged before. After it's merge, rebase and check if all is still working.

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