有两种方法：
1.在AppDelegate中添加
[[UITabBar appearance] setShadowImage:[[UIImage alloc]init]];
[[UITabBar appearance] setBackgroundImage:[[UIImage alloc]init]];
2.在tabbarVC中添加
其实并没有删除横线（remove掉），只是把它变成透明的不影响操作和界面美观而已（视觉错）
CGRect rect = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
UIGraphicsBeginImageContext(rect.size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(context, [UIColor clearColor].CGColor);
CGContextFillRect(context, rect);
UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[self.tabBar setBackgroundImage:img];
[self.tabBar setShadowImage:img];
