去除导航栏返回按钮UIBarButtonItem

这个方法用在返回按钮是自定义的时候

UIView *emptyView = [[UIView alloc] init];;
UIBarButtonItem *emptyButton = [[UIBarButtonItem alloc] initWithCustomView:emptyView];
    
    [self.navigationItem setLeftBarButtonItem:emptyButton animated:YES];
    ```
## 这个方法只有在返回按钮是系统的时候才有效果

[self.navigationItem setHidesBackButton:YES animated:YES];