Removed unnecessary TODO and code duplication

src/Umbraco.Infrastructure/Install/InstallSteps/NewInstallStep.cs

@@ -61,7 +61,6 @@ namespace Umbraco.Web.Install.InstallSteps
 
             _userService.Save(admin);
 
-            //TODO: This needs to be reintroduced, when users are compatible with ASP.NET Core Identity.
             var membershipUser = await _userManager.FindByIdAsync(Constants.Security.SuperUserId.ToString());
             if (membershipUser == null)
             {

src/Umbraco.Web.BackOffice/Extensions/UmbracoBackOfficeServiceCollectionExtensions.cs

@@ -24,7 +24,6 @@ namespace Umbraco.Extensions
 
             services.TryAddScoped<IIpResolver, AspNetCoreIpResolver>();
 
-            services.AddIdentityCore<BackOfficeIdentityUser>();
             services.AddIdentityCore<BackOfficeIdentityUser>(options =>
                 {
                     options.User.RequireUniqueEmail = true;