Sliding expiration for auth cookie and a little clean up

Fixes #1802

(cherry picked from commit 05ee4e644907b7f1e84589465ac9ab1848f5a766)

src/Readarr.Http/Authentication/AuthenticationBuilderExtensions.cs

@@ -33,6 +33,7 @@ namespace Readarr.Http.Authentication
                     options.AccessDeniedPath = "/login?loginFailed=true";
                     options.LoginPath = "/login";
                     options.ExpireTimeSpan = TimeSpan.FromDays(7);
+                    options.SlidingExpiration = true;
                 })
                 .AddApiKey("API", options =>
                 {

src/Readarr.Http/Authentication/AuthenticationService.cs

@@ -15,17 +15,14 @@ namespace Readarr.Http.Authentication
 
     public class AuthenticationService : IAuthenticationService
     {
-        private const string AnonymousUser = "Anonymous";
         private static readonly Logger _authLogger = LogManager.GetLogger("Auth");
         private readonly IUserService _userService;
 
-        private static string API_KEY;
         private static AuthenticationType AUTH_METHOD;
 
         public AuthenticationService(IConfigFileProvider configFileProvider, IUserService userService)
         {
             _userService = userService;
-            API_KEY = configFileProvider.ApiKey;
             AUTH_METHOD = configFileProvider.AuthenticationMethod;
         }