DDS4CCM11 — Remove key_fields

We propose to remove key_fields completely from the DDS_TopicBase connector. The knowledge which fields are a key is specified in IDL using annotations and shouldn't be set again on the connector instance. This is duplicate information which could only lead to inconsistencies and possible problems. Until now we also have not seen any use case which require this information to be available as StringSeq as part of a connector instance.