[REF] fetchmail: clean and remove unnecessary code about mail.mail

Purpose: small cleaning of code linked to mail.mail model in order to
avoid unnecessary overrides

  * create mail: default_fetchmail_server_id in context should be sufficient
    as it is the base mechanism of default_get;
  * write: unused in codebase. Anyway we should not write on mail and if it
    is the case giving fetchmail_server_id directly should be the correct
    way of doing it;

In case of a clean_context use we might loose default_fetchmail_server_id
key. It is used notably for tracking message generation. In that case
this information could be lost. However the whole concept of propagating
a fetrchmail_server-id from incoming mail server to outgoing emails (as
mail.mail is used only for sending email) is strange and does not require
so much specific code.

Sub-part of task 1853147 (pre-cleaning before implementing mail gateway
improvements)
Linked to PR #32974

addons/fetchmail/models/fetchmail.py

@@ -160,7 +160,7 @@ class FetchmailServer(models.Model):
         MailThread = self.env['mail.thread']
         for server in self:
             _logger.info('start checking for new emails on %s server %s', server.server_type, server.name)
-            additionnal_context['fetchmail_server_id'] = server.id
+            additionnal_context['default_fetchmail_server_id'] = server.id
             additionnal_context['server_type'] = server.server_type
             count, failed = 0, 0
             imap_server = None

addons/fetchmail/models/mail_mail.py

@@ -1,25 +1,10 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
-from odoo import api, fields, models
+from odoo import fields, models
 
 
 class MailMail(models.Model):
-
-    _inherit = "mail.mail"
+    _inherit = 'mail.mail'
 
     fetchmail_server_id = fields.Many2one('fetchmail.server', "Inbound Mail Server", readonly=True, index=True, oldname='server_id')
-
-    @api.model
-    def create(self, values):
-        fetchmail_server_id = self.env.context.get('fetchmail_server_id')
-        if fetchmail_server_id:
-            values['fetchmail_server_id'] = fetchmail_server_id
-        return super(MailMail, self).create(values)
-
-    @api.multi
-    def write(self, values):
-        fetchmail_server_id = self.env.context.get('fetchmail_server_id')
-        if fetchmail_server_id:
-            values['fetchmail_server_id'] = fetchmail_server_id
-        return super(MailMail, self).write(values)