[FIX] purchase: remove useless and broken override

There was several issues with this method:
- the workflow is incorrectly called (`purchase.signal_workflow(...)` instead)
- the workflow no longer exists (`purchase.state = 'sent'` instead)
- the code is only triggered when sending an actual mail and not a notification
- the code is never executed as the change is already changed

The last point is due to the override of `send_mail` in mail.compose.message.
The state is changed before `_postprocess_sent_message` is called (if called at
all).
Similar change of state is already done for sale.order.

This commit replaces and closes #14760

addons/purchase/models/__init__.py

@@ -2,7 +2,6 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 import account_invoice
-import mail_mail
 import purchase
 import res_company
 import res_config

addons/purchase/models/mail_mail.py

@@ -1,18 +0,0 @@
-# -*- coding: utf-8 -*-
-# Part of Odoo. See LICENSE file for full copyright and licensing details.
-
-from odoo import api, models
-
-
-class MailMail(models.Model):
-    _name = 'mail.mail'
-    _inherit = 'mail.mail'
-
-    @api.multi
-    def _postprocess_sent_message(self, mail_sent=True):
-        for mail in self:
-            if mail_sent and mail.model == 'purchase.order':
-                purchase_order = self.env['purchase.order'].browse(mail.res_id)
-                if purchase_order.state == 'draft':
-                    self.env['purchase.order'].signal_workflow([mail.res_id], 'send_rfq')
-        return super(MailMail, self)._postprocess_sent_message(mail_sent=mail_sent)