API: add `inplace` kwarg to NDFrame.droplevel
```python
df.droplevel(level, axis=0)
```

vs

```python
df.droplevel(level, axis=0, inplace=False)
```

#### Problem description

Currently `.droplevel` DataFrame / Series does not expose `inplace` kwarg, and it defaults to False under the hood, in `NDFrame.set_axis` method

cc @jreback @TomAugspurger 