simplify post box preset

dropping the GB regional variant in favour of making the `royal_cypher` field regional.

see also https://github.com/ideditor/schema-builder/issues/94

data/fields/royal_cypher-GB.json

@@ -16,5 +16,10 @@
         }
     },
     "autoSuggestions": false,
-    "customValues": false
+    "customValues": false,
+    "locationSet": {
+        "include": [
+            "gb"
+        ]
+    }
 }