delete link when done

After disconnecting instances from links, delete the link.
This was discovered by running a pyxtuml model which did
many creates and relates and then unrelates and deletes.

Author: cortlandstarrett

xtuml/meta.py

@@ -335,6 +335,8 @@ def disconnect(self, instance, another_instance):
             return False
 
         self[instance].remove(another_instance)
+        if len(self[instance]) == 0:
+            del self[instance]
         return True
 
     def navigate(self, instance):