[Removed the JSON library] (#816)

Removed the JSON dll and associated meta files
Removed references to JSON libraries in the GridwWorld Agent script

unity-environment/Assets/ML-Agents/Examples/GridWorld/Scripts/GridAgent.cs

-using System.Collections;
-using System.Collections.Generic;
-using UnityEngine;
-
-using UnityEngine.UI;
+using UnityEngine;
-using Newtonsoft.Json;
-using System.Net;
-using System.Net.Sockets;
-using System.Text;
 
 public class GridAgent : Agent
 {

unity-environment/Assets/ML-Agents/Plugins/JSON.meta

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-guid: 19240955c74d74ddf87ba8e888965f7b
-folderAsset: yes
-timeCreated: 1504198536
-licenseType: Free
-DefaultImporter:
-  userData: 
-  assetBundleName: 
-  assetBundleVariant: