Also properly delete values here

DeukBot4/Database/BirthdayHandler.cs

@@ -129,6 +129,7 @@ namespace DeukBot4.Database
                     {
                         Console.WriteLine("Can't celebrate birthday for user, not in guild anymore :(");
                         db.KeyDelete(val);
+                        db.SetRemove(birthday.ToString("ddMM"), val);
                         return;
                     }