Remove convience model artifacts

Artifacts like rfc6991 do not contribute anything to the actual picture and act only as maven dependency shortcut. In that role they are defeated by maven-dependency-plugin, as that reports the components (ietf-inet-types and ietf-yang-types) as dependencies.

This is much more pronounced with JPMS: downstream module have to declare a module dependency on the actual code artifacts anyway.

Eliminate all of these artifacts, so that downstreams have to explicitly define what they depend on.