How can you remove default fields from a content entity

Using Drupal console I generated a content entity I started to remove the fields defined (user_id, language etc etc) as these aren't required. However, when I attempted to save the some data using the add form generated I got the following error: Uncaught PHP Exception InvalidArgumentException: "Field changed is unknown." at /var/www/html/core/lib/Drupal/Core/Entity/ContentEntityBase.php line 474. As this is the first content entity I have created I am quite confused by what I've done wrong.