identity: simplify user sign up

Notesnook.API/Models/Responses/SignupResponse.cs

@@ -1,14 +0,0 @@
-using System.Text.Json.Serialization;
-using Streetwriters.Common.Models;
-
-namespace Notesnook.API.Models.Responses
-{
-    public class SignupResponse : Response
-    {
-        [JsonPropertyName("userId")]
-        public string? UserId { get; set; }
-
-        [JsonPropertyName("errors")]
-        public string[]? Errors { get; set; }
-    }
-}