AdminMetadata class has 3 subclasses that look like mistakes

Kirk Hess pointed out that AdminMetadata is subClassed As:  DescriptionConventions, GenerationProcess, and DescriptionAuthentication

Those are classes, but not subclasses of AdminMetadata. We should remove 
```xml
<rdfs:subClassOf rdf:resource="http://id.loc.gov/ontologies/bibframe/AdminMetadata"/>
```

from each.
See:
https://id.loc.gov/ontologies/bibframe/DescriptionConventions.rdf