The mail code had to override both new and old form views to add the chatter feature. Since we converted (almost) all uses of the legacy form view to the new owl views, the old override is no longer necessary. This commit removes it, and also removes the 'legacy_form' entry in the registry so all screens should fall back to the new form view, which is stable. closes odoo/odoo#110168 Related: odoo/enterprise#35924 Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>