LightShadows.None  

// Set light to not cast shadows.

light.shadows = LightShadows.None;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
light.shadows = LightShadows.None;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
light.shadows = LightShadows.None