Remove unused message id

Ooui/Message.cs

@@ -7,9 +7,6 @@ namespace Ooui
 {
     public class Message
     {
-        [JsonProperty("mid")]
-        public long Id = GenerateId ();
-
         [JsonProperty("m")]        
         public MessageType MessageType = MessageType.Nop;
 
@@ -19,7 +16,7 @@ namespace Ooui
         [JsonProperty("k")]
         public string Key = "";
 
-        [JsonProperty("v")]
+        [JsonProperty("v", NullValueHandling = NullValueHandling.Ignore)]
         public object Value = null;
 
         public static Message Call (string targetId, string method, params object[] args) => new Message {
@@ -34,12 +31,6 @@ namespace Ooui
             TargetId = targetId,
             Key = eventType,
         };
-
-        static long idCounter = 0;
-        static long GenerateId ()
-        {
-            return System.Threading.Interlocked.Increment (ref idCounter);
-        }
     }
 
     [JsonConverter (typeof (StringEnumConverter))]