*: mail_plugin, crm_mail_plugin, project_mail_plugin

The code that is discovering translatable strings is defined in
[translate.py]. It does lookup in all python files and in all javascript
files defined under 'static/src/'.

The top-level `web.assets_backend` directive in the manifest is useless
and have been removed.

[translate.py]: https://github.com/odoo/odoo/blob/8b468e1f7a3dfeb2bcfa83bff55c4b5adf6b2212/odoo/tools/translate.py#L1138-L1147

Part-of: odoo/odoo#79977