catz: unregister the db update-notify callback before detaching from db

catz: unregister the db update-notify callback before detaching from db

When detaching from the previous version of the database, make sure
that the update-notify callback is unregistered, otherwise there is
an INSIST check which can generate an assertion failure in free_rbtdb(),
which checks that there are no outstanding update listeners in the list.

There is a similar code already in place for RPZ.

(cherry picked from commit cf79692a662b627d2230fbf7ae67f55cf729180c)