方式一：

self.navigationController.navigationBar.subviews[0].subviews[0].hidden = YES;

方式二：

UIImageView *navBarHairlineImageView = [self findLineImageViewUnder:self.navigationController.navigationBar];
navBarHairlineImageView.hidden = YES;

- (UIImageView *)findLineImageViewUnder:(UIView *)view {
    if ([view isKindOfClass:UIImageView.class] && view.bounds.size.height <= 1.0)
     {
        return (UIImageView *)view;
    }
    for (UIView *subview in view.subviews) {
        UIImageView *imageView = [self findLineImageViewUnder:subview];
        if (imageView) {
            return imageView;
        }
    }
    return nil;
}