The mail app no longer uses an older form screen customization for chatter, because the newer form views now cover the needed behavior. This reduces maintenance complexity and helps ensure screens consistently use the stable modern form experience.

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:




---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

This change removes legacy code that added chatter to older form views in the Mail Enterprise module. It simplifies maintenance because that compatibility layer is no longer useful in practice and should not affect normal current usage.