Fixing a handful of warnings that showed up since the upgrade to .NET Core 2 (#411)

src/Microsoft.SqlTools.ServiceLayer/Admin/Common/ICloneable.cs

@@ -1,12 +0,0 @@
-//
-// Copyright (c) Microsoft. All rights reserved.
-// Licensed under the MIT license. See LICENSE file in the project root for full license information.
-//
-
-namespace System
-{
-    internal interface ICloneable
-    {
-        object Clone();
-    }
-}