remove_action on after_setup_theme not working from child theme

As @cybmeta already pointed out, you are too early with your removal. So you have to defer the actual removal, for instance like so:

add_action( 'after_setup_theme', 'wpdev_170663_remove_parent_theme_stuff', 0 );

function wpdev_170663_remove_parent_theme_stuff() {

    remove_action( 'after_setup_theme', 'et_pb_setup_theme' );
}