[FIX] website_mail: do not propagate csrf_token to message_post method

Indeed through portal users can post a message. Form used in order to
do so include a csrf token. However this one should not be propagated
to message_post because it is not a mail_message model field.
This creates unnecessary warnings.

Closes #13956 .

addons/website_mail/controllers/main.py

@@ -65,6 +65,7 @@ def _message_post_helper(res_model='', res_id=None, message='', token='', token_
             res = res.sudo()
         else:
             raise NotFound()
+    kw.pop('csrf_token', None)
     return res.with_context({'mail_create_nosubscribe': nosubscribe}).message_post(body=message,
                                                                                    message_type=kw.pop('message_type', False) or "comment",
                                                                                    subtype=kw.pop('subtype', False) or "mt_comment",