Revert "Merge pull request #13604 from Jxiced/master"

This reverts commit ce78af2ed4, reversing
changes made to db7465e83d.

MediaBrowser.Controller/Library/IUserManager.cs

@@ -33,12 +33,6 @@ namespace MediaBrowser.Controller.Library
         /// <value>The users ids.</value>
         IEnumerable<Guid> UsersIds { get; }
 
-        /// <summary>
-        /// Checks if the user's username is valid.
-        /// </summary>
-        /// <param name="name">The user's username.</param>
-        void ThrowIfInvalidUsername(string name);
-
         /// <summary>
         /// Initializes the user manager and ensures that a user exists.
         /// </summary>