$.ajax({
    type: "GET",
    url: '/cart.js',
    dataType: 'json',
    success: function(resp) { 
        var nullHash = {};
        for ([key, value] of Object.entries(resp.attributes)) {    
            nullHash[key] = null;
        }
        $.ajax({
            type: "POST",
            url: '/cart/update.js',
            data: {attributes:nullHash},
            dataType: 'json'
        });
    }
});
Or if you want to delete them by name, you can do this:

var nullHash = {
    'someAtt1': null,
    'someAtt2': null,
    'someAtt3': null
};
$.ajax({
    type: "POST",
    url: '/cart/update.js',
    data: {attributes:nullHash},
    dataType: 'json'
});