fix: do not update LMS user during retirement

[Agrendalath]

1. Modifying users is already handled by the LMS retirement pipeline. The forum library should not alter LMS users during retirement.
2. Changing the email to an empty string results in integrity errors in the MySQL backend, because the email must be unique.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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