Core Service: methods for deleting types (#607).

2 methods added to Core Service:
{{{
Directives deleteTopicType(String topicTypeUri)
Directives deleteAssociationType(String assocTypeUri)
}}}
Deletes the respective type from the DB and the type cache.
The directives returned include a DELETE_TOPIC_TYPE resp. DELETE_ASSOCIATION_TYPE directive.

REST API:
{{{
DELETE /core/topictype/{uri}
DELETE /core/assoctype/{uri}
}}}

See #607.