delete dl:DiagramObjectGluePoint

Delete this useless shape in 61970-600-2_DiagramLayout-AP-Con-Simple-SHACL.ttl, which does nothing:
```ttl
dl:DiagramObjectGluePoint
        rdf:type        sh:NodeShape ;
        sh:targetClass  cim:DiagramObjectGluePoint .
```
Found through https://github.com/Sveino/Inst4CIM-KG/tree/develop/shacl-improved#each-nodeshape-should-have-a-property