UINavigationBar 去除底部黑线

获取_UIBarBackgroundShadowView，通过获取到的UIView来设置隐藏/显示

[self.navigationController.navigationBar.subviews.firstObject.subviews enumerateObjectsUsingBlock:^(__kindof UIView * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
            if ([obj isKindOfClass:NSClassFromString(@"_UIBarBackgroundShadowView")]) {
                obj.hidden=YES;
            }
        }];