Core: add remove() to attached comp value (#339).

AttachedCompositeValue:
    - remove(String childTypeUri, long topicId, ..) is new method

It removes a child topic from multiple-value childs.
(Analogue to remove() in CompositeValueModel).

Can be used for both:
    - Composition: the child topic is deleted.
    - Aggregation: only the assignment (association) is deleted.

See ticket 339.