I think the problem is position of the variable "polyline".
var polyline = null;
downloadUrl("myfile.asp", function(data) {
...
polyline = new google.maps.Polyline({
path: path,
strokeColor: "#FF0000",
strokeOpacity: 1.0,
strokeWeight: 2
});
polyline.setMap(map);
}); //end download url
function removePath() {
polyline.setMap(null)
}