$gateways = WC_Payment_Gateways::instance(); // gateway instance
$available_gateways = $gateways->get_available_payment_gateways();


if ( isset( $available_gateways['cod'] ) )
    remove_action( 'woocommerce_email_before_order_table', array( $available_gateways['cod'], 'email_instructions' ), 10, 3 );