How to remove the images from “Uploaded to this post”?

You have to unattach the images from the post.
And that means setting their 'post_parent' => 0 in wp_posts database table.

The plugin Unattach and Re-attach Media Attachments does exactly this, and it uses the following command to this end:

$wpdb->update(
    $wpdb->posts, 
    array( 'post_parent' => 0 ),
    array(
        'id' => (int)$_REQUEST['post_id'], 
        'post_type' => 'attachment'
    )
);