[FIX] *mail_plugin: unused i18n path in manifest

*: 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

addons/crm_mail_plugin/__manifest__.py

@@ -17,9 +17,6 @@
         'views/crm_mail_plugin_lead.xml',
         'views/crm_lead_views.xml'
     ],
-    'web.assets_backend': [
-        'crm_mail_plugin/static/src/to_translate/**/*',
-    ],
     'installable': True,
     'application': False,
     'auto_install': True,

addons/mail_plugin/__manifest__.py

@@ -13,9 +13,6 @@
         'contacts',
         'iap'
     ],
-    'web.assets_backend': [
-        'mail_plugin/static/src/to_translate/**/*',
-    ],
     'data': [
         'views/mail_plugin_login.xml',
         'views/res_partner_iap_views.xml',

addons/project_mail_plugin/__manifest__.py

@@ -12,9 +12,6 @@
         'views/project_task_views.xml'
     ],
     'website': 'https://www.odoo.com/app/project',
-    'web.assets_backend': [
-        'project_mail_plugin/static/src/to_translate/**/*',
-    ],
     'depends': [
         'project',
         'mail_plugin',