Remove legacy auth code (#1677)

* Remove legacy auth code

* Adds tests so we don't break PasswordHash (again)
* Clean up interfaces
* Remove duplicate code

* Use auto properties

* static using

* Don't use 'this'

* Fix build

MediaBrowser.Common/Extensions/HexHelper.cs

@@ -1,22 +0,0 @@
-using System;
-using System.Globalization;
-
-namespace MediaBrowser.Common.Extensions
-{
-    public static class HexHelper
-    {
-        public static byte[] FromHexString(string str)
-        {
-            byte[] bytes = new byte[str.Length / 2];
-            for (int i = 0; i < str.Length; i += 2)
-            {
-                bytes[i / 2] = byte.Parse(str.Substring(i, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture);
-            }
-
-            return bytes;
-        }
-
-        public static string ToHexString(byte[] bytes)
-            => BitConverter.ToString(bytes).Replace("-", "");
-    }
-}