Remove the test migrate assets

which is used only in migrate tests

TestAssets/DesktopTestProjects/AutoAddDesktopReferencesDuringMigrate/Program.cs

@@ -1,24 +0,0 @@
-// Copyright (c) .NET Foundation and contributors. All rights reserved.
-// Licensed under the MIT license. See LICENSE file in the project root for full license information.
-
-using System.Collections.Generic;
-using System.Diagnostics;
-using System.Linq;
-using Microsoft.CSharp.RuntimeBinder;
-
-namespace AutoAddDesktopReferencesDuringMigrate
-{
-    class Program
-    {
-        static void Main(string[] args)
-        {
-            var mscorlibRef = new List<int>(new int[] { 4, 5, 6 });
-            var systemCoreRef = mscorlibRef.ToArray().Average();
-            Debug.Assert(systemCoreRef == 5, "Test System assembly reference");
-            if (systemCoreRef != 5)
-            {
-                throw new RuntimeBinderException("Test Microsoft.CSharp assembly reference");
-            }
-        }
-    }
-}

TestAssets/DesktopTestProjects/AutoAddDesktopReferencesDuringMigrate/project.json

@@ -1,23 +0,0 @@
-{
-  "version": "1.0.0-*",
-  "buildOptions": {
-    "debugType": "portable",
-    "emitEntryPoint": true
-  },
-  "frameworks": {
-    "net451": {}
-  },
-  "runtimes": {
-    "win7-x64": {},
-    "win7-x86": {},
-    "osx.10.10-x64": {},
-    "osx.10.11-x64": {},
-    "ubuntu.14.04-x64": {},
-    "ubuntu.16.04-x64": {},
-    "centos.7-x64": {},
-    "rhel.7.2-x64": {},
-    "debian.8-x64": {},
-    "fedora.23-x64": {},
-    "opensuse.13.2-x64": {}
-  }
-}