[IMP] mail: remove useless menu cache invalidation

Rev. 15174e9a70 explicitly
cached ir.ui.menu entries, and required an explicit
cache invalidation when users changed their mail.group
subscriptions (mail.group used to override the ir.ui.menu
search(), cfr e.g. 89e404301b)

The menu search() override is gone as of 9.0, so it's time we get
rid of the unnecessary cache invalidation.

addons/mail/models/mail_thread.py

@@ -1895,10 +1895,7 @@ class MailThread(models.AbstractModel):
             provided, subscribe uid instead. """
         if user_ids is None:
             user_ids = [self._uid]
-        result = self.message_subscribe(self.env['res.users'].browse(user_ids).mapped('partner_id').ids, subtype_ids=subtype_ids)
-        if user_ids and result:
-            self.pool['ir.ui.menu'].clear_caches()
-        return result
+        return self.message_subscribe(self.env['res.users'].browse(user_ids).mapped('partner_id').ids, subtype_ids=subtype_ids)
 
     @api.multi
     def message_subscribe(self, partner_ids=None, channel_ids=None, subtype_ids=None, force=True):
@@ -1939,10 +1936,7 @@ class MailThread(models.AbstractModel):
         if user_ids is None:
             user_ids = [self._uid]
         partner_ids = [user.partner_id.id for user in self.env['res.users'].browse(user_ids)]
-        result = self.message_unsubscribe(partner_ids)
-        if partner_ids and result:
-            self.pool['ir.ui.menu'].clear_caches()
-        return result
+        return self.message_unsubscribe(partner_ids)
 
     @api.multi
     def message_unsubscribe(self, partner_ids=None, channel_ids=None):