[FIX] Force safer ondelete=restrict on the `alias_id` _inherits FK on models with a mail.alias parent

The delete-rule was initially set to `cascade` in revision
6897.16.13 revid:odo@openerp.com-20120705141706-5gm5mmqode3bvkuc
because the ORM would not allow anything else for the FK of
an _inherits relationship.
This constraint was later lifted in 7.0 server at revision
4681 revid:odo@openerp.com-20121212210247-emrz5rf9ewcwdggu
so we can now switch to the intended default behavior:
when deleting mail aliases we never want to cascade delete
the child records, as that could lead to unwanted deletions.
On the other hand the aliases are automatically deleted when
the record they belong to is deleted, as a kind of internal
dependency. This is the intended safe way to delete them.

There is a special case when the same alias is manually set
on multiple records, in which case you will not be able to
delete any of those records. This is an acceptable exception
and should be manually handled if ever needed, by temporarily
linking the records to delete to new dummy aliases.