Remove the u2f lib (#1820)

src/Core/Entities/U2f.cs

@@ -1,26 +0,0 @@
-using System;
-using System.ComponentModel.DataAnnotations;
-
-namespace Bit.Core.Entities
-{
-    public class U2f : ITableObject<int>
-    {
-        public int Id { get; set; }
-        public Guid UserId { get; set; }
-        [MaxLength(200)]
-        public string KeyHandle { get; set; }
-        [MaxLength(200)]
-        public string Challenge { get; set; }
-        [MaxLength(50)]
-        public string AppId { get; set; }
-        [MaxLength(20)]
-        public string Version { get; set; }
-        public DateTime CreationDate { get; internal set; } = DateTime.UtcNow;
-
-        public void SetNewId()
-        {
-            // int will be auto-populated
-            Id = 0;
-        }
-    }
-}