去掉tabBar黑線， iOS13

iOS 13 之前， 去掉黑線：設置UITarBar的 backgroundImage 和 shadowImage即可：

        tabbar.backgroundImage = UIImage(color: UIColor.clear)
        tabbar.shadowImage = UIImage(color: UIColor.clear)

iOS13之後不好使了，改用下面的方法：

        let appearance = tabbar.standardAppearance.copy()
        appearance.backgroundImage = UIImage(color: UIColor.clear)
        appearance.shadowImage = UIImage(color: UIColor.clear)
        tabbar.standardAppearance = appearance

所以兼容代碼如下：

    if #available(iOS 13, *) {
        let appearance = tabbar.standardAppearance.copy()
        appearance.backgroundImage = UIImage(color: UIColor.clear)
        appearance.shadowImage = UIImage(color: UIColor.clear)
        tabbar.standardAppearance = appearance
    } else {
        tabbar.backgroundImage = UIImage(color: UIColor.clear)
        tabbar.shadowImage = UIImage(color: UIColor.clear)
    }