RATIONALE Channel model is a mail.thread enabled model behaving strangely with followers, notifications and discuss. Its code should however be simplified to be more self contained and avoid unwanted side effects on other models. SPECIFICATIONS Remove ``message_channel_ids`` field from ``mail.thread``. As we removed support of (un)subscribing channel-based followers there is no need anymore to have a field to access them. We can now safely remove this field as it has no use anymore. LINKS Task ID-2070632 (main task) Task ID-2419762 (followup task) COM PR odoo/odoo#62859 ENT PR odoo/enterprise#15172 UPG PR odoo/upgrade#2005