如何去掉Navigation下方的分割线

if ([self.navigationController.navigationBar respondsToSelector:@selector( setBackgroundImage:forBarMetrics:)]){
NSArray *list=self.navigationController.navigationBar.subviews;
for (id obj in list) {
if ([obj isKindOfClass:[UIImageView class]]) {
UIImageView *imageView=(UIImageView *)obj;
NSArray *list2=imageView.subviews;
for (id obj2 in list2) {
if ([obj2 isKindOfClass:[UIImageView class]]) {
UIImageView *imageView2=(UIImageView *)obj2;
imageView2.hidden=YES;
}
}
}
}
}