Removing unused interface

Fixes: https://github.com/dotnet-architecture/eShopOnWeb/issues/304

src/ApplicationCore/Interfaces/IIdentityParser.cs

@@ -1,9 +0,0 @@
-using System.Security.Principal;
-
-namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
-{
-    public interface IIdentityParser<T>
-    {
-        T Parse(IPrincipal principal);
-    }
-}