Hide the Discount Code field on Easy Digital Downloads checkout.

my_edd_remove_discount_field.php

<?php
/**
 * Hide the Discount Code field on Easy Digital Downloads checkout.
 * Customers can only apply coupons using a URL attribute like:
 * http://sitewidesales.local/checkout/?discount=loyal.
 */
function my_edd_no_discount_code_on_checkout() {
	remove_action( 'edd_checkout_form_top', 'edd_discount_field', -1 );
}
add_action( 'init', 'my_edd_no_discount_code_on_checkout' );