This inheritance wasn't working till the commit of rco, modifying
models inheritance.
Anyway, `website_message_ids` field is already defined on several
models (product, slide, forum questions, blog post, ...), and each
of them have a different domain attribute.
So making the mail.thread mixin providing this field is not so
interessting since it will be overrided for specific behavior.
We remove this field as it can't be usefull and generic.
Jérome Maes
fb88cff1bd