Drop redundant index and default value for merge_request_diff_commit_users

What does this MR do and why?

This change cleans up the database structure for tracking commit authors in merge requests. It removes a redundant database index that was no longer needed (since there's already a more comprehensive index that serves the same purpose), and removes a default value from the organization_id column. These are maintenance changes that simplify the database schema without affecting functionality - like removing duplicate filing systems and cleaning up outdated default settings.

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Related to #526298 (closed) and #517135 (closed)