When removing a custom model, it remained listed in the base classes
(normally `base` and also possibly a list of mixins; e.g. `mail.thread`
or `mail.activity.mixin`) list of inheriting classes, possibly causing a
crash when trying to reload the registry.
This commit ensures that any custom model is removed from its parent
class `_inherit_children` set; it will be re-added automatically during
the call to `_build_model` if the custom model still exists.
Co-Authored-By: Raphaël Collet <rco@odoo.com>
