prefix had unneeded key

Author: ddooley

web/templates/schema_editor/schema.json

@@ -396,8 +396,7 @@
           "unique_key_name": "prefix_key",
           "unique_key_slots": [
             "schema_id",
-            "prefix",
-            "reference"
+            "prefix"
           ],
           "description": "A slot is uniquely identified by the schema it appears in as well as its name"
         }

web/templates/schema_editor/schema.yaml

@@ -96,7 +96,6 @@ classes:
         unique_key_slots:
           - schema_id
           - prefix
-          - reference
     slots:
       - schema_id
       - prefix

web/templates/schema_editor/schema_core.yaml

@@ -32,7 +32,6 @@ classes:
         unique_key_slots:
         - schema_id
         - prefix
-        - reference
 
   Class:
     name: Class