取消navigationLeftItem返回两个字

方法一, 感谢http://www.jianshu.com/p/457c80cbb487

self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStylePlain target:nil action:nil];

方法二
感谢http://www.jianshu.com/p/4523eafb4cd4

[[UIBarButtonItem appearance] setBackButtonTitlePositionAdjustment:UIOffsetMake(0, -60) forBarMetrics:UIBarMetricsDefault];