https://learn.javascript.ru/task/remove-class

var removeClass = function(obj, cls) {
    var mas = obj.className.split(' ');
    if (mas.indexOf(cls) != -1) {
        var a = mas.indexOf(cls);
        mas.splice(a, 1);
    }
    obj.className = mas.join(' ');
};

var obj = {
className: 'my menu menu'
};

removeClass(obj, 'menu');

alert(obj.className);