Confirm why we need push rules validation in before merge and if we can remove it

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In #467506 (comment 2013134893), we found out that we are also validating commit message if they pass push rules before merging.

It's not clear why we do this since we are also checking the same thing in pre-receive hook once the actual merge occurs. So we need to find out why and see if we can remove it and what are the effects of doing so.

This may require digging into the history of it and trying out what happens on UI, GraphQL Accept mutation, Accept MR REST API endpoint, auto merge (MWCP, Merge train), and quick action merge if we remove it.