How do I remove all the metaboxes for a custom post type?

If you want to hide all the metaboxes for all users, you can do that through the get_user_option_metaboxhidden_{cpt}, default_hidden_meta_boxes or the hidden_meta_boxes filter, according to the /wp-admin/includes/screen.php file.

Here’s an example for the post post type:

/**
 * Hide all metaboxes in the global $wp_meta_boxes
 */

add_filter( 'hidden_meta_boxes', function( $hidden, $screen, $use_defaults )
{
    global $wp_meta_boxes;
    $cpt="post"; // Modify this to your needs!

    if( $cpt === $screen->id && isset( $wp_meta_boxes[$cpt] ) )
    {
        $tmp = array();
        foreach( (array) $wp_meta_boxes[$cpt] as $context_key => $context_item )
        {
            foreach( $context_item as $priority_key => $priority_item )
            {
                foreach( $priority_item as $metabox_key => $metabox_item )
                    $tmp[] = $metabox_key;
            }
        }
        $hidden = $tmp;  // Override the current user option here.
    }
    return $hidden;
}, 10, 3 );