[FIX] mail: fix discuss.channel sending two times push notification (backport)

Backport of odoo/odoo@f312762c6a

'_notify_thread' calls '_notify_thread_by_web_push'. There is therefore no
need to call it once again in discuss.channel override of '_notify_thread'.

Task-3695571
Related to Task-3669738 (Mail: Web Push Models Rename and Fixes)

Part-of: odoo/odoo#150011

addons/mail/models/discuss/discuss_channel.py

@@ -556,8 +556,6 @@ class Channel(models.Model):
         ]
         # sudo: bus.bus - sending on safe channel (discuss.channel)
         self.env["bus.bus"].sudo()._sendmany(bus_notifications)
-        if self.is_chat or self.channel_type == "group":
-            self._notify_thread_by_web_push(message, rdata, msg_vals, **kwargs)
         return rdata
 
     def _message_receive_bounce(self, email, partner):