Remove page refresh browser popup

Question

I added page beforeunload and unload functions as follows:

window.onunload = function (e) 
                {
                    //mycode1
                    return '';
                };
window.onbeforeunload = function(e){
                    //mycode2
                    return '';
                }

and they are getting called after browser confirmation popup occurs. But now I want to unbind these functions. I tried

window.onunload = null;
window.onbeforeunload = null;

But seems like they are not working and still browser refresh confirmation popup appears.