Unnecessary 3D checking in _plot_img_with_bg

Unless there is something I am missing, the 3D checking is done already on [l.47](https://github.com/lesteve/nilearn/blob/380df8a08f2014ce22125853edcdb064097e1fd6/nilearn/plotting/img_plotting.py#L47) through:
```python
img = _utils.check_niimg(img, ensure_3d=True)
```