NBUCameraViewController: Do not remove `lastPictureImageView` on single image mode
NBUCameraViewController: Do not remove `lastPictureImageView` on single image mode

CyberAgent/iOS-NBUImagePicker,dvan001/iOS-NBUImagePicker,jonesgithub/iOS-NBUImagePicker,dvan001/iOS-NBUImagePicker,CyberAgent/iOS-NBUImagePicker,jonesgithub/iOS-NBUImagePicker,dvan001/iOS-NBUImagePicker,dvan001/iOS-NBUImagePicker,jonesgithub/iOS-NBUImagePicker,jonesgithub/iOS-NBUImagePicker