In preparation to clean code of models, e.g. by simplifying model insertion and reducing side-effects in record insertion. This commit moves the implementation of the `insert()` in discuss services to the models, and drop support of the service methods `insert()`. The only way to insert a model is through the store entry, e.g. `store.Thread.insert()`. closes odoo/odoo#133509 Related: odoo/enterprise#46499 Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>