remove unused method in map converter

Author: rose-a

src/GraphQL.Client.Serializer.Newtonsoft/MapConverter.cs

@@ -61,8 +61,6 @@ private IEnumerable<object> ReadArray(JToken element)
             }
         }
 
-        private object ReadNumber(JToken token) => ((JValue) token).Value;
-
         private bool IsUnsupportedJTokenType(JTokenType type) => type == JTokenType.Constructor || type == JTokenType.Property || type == JTokenType.Comment;
     }
 }