Remove old and useless district diagram's own diagram layer contributions
Originally, back when the Simantics diagram layerin was not working at all, Jani tried to add separate layering for district network diagrams.
In the end the code turned out un-useful and those contributions should now be removed as much as possible since the current Simantics trunk has resurrected the diagram layer system (platform#566 (closed)).
The ontology contributions must be mainly left in place because models could no longer be imported if they were all removed. However, we should mark the related ontology concepts deprecated.