Remove unused namespace decl

public/instance/schema/diglib-rightsmd.rnc

 # Administrative Metadaten
 #
 # Autor: David Maus <maus@hab.de>
-# Timestamp: <2019-01-16 12:17:50 maus>
+# Timestamp: <2019-01-16 12:19:33 maus>
 #
 
 namespace dct = "http://purl.org/dc/terms/"
@@ -9,7 +9,6 @@ namespace foaf = "http://xmlns.com/foaf/0.1/"
 namespace marcrel = "http://id.loc.gov/vocabulary/relators/"
 namespace owl = "http://www.w3.org/2002/07/owl#"
 namespace rdf = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-namespace skos = "http://www.w3.org/2004/02/skos/core#"
 
 include "shared-dc.rnc"