CLN: removed unused parameter 'out' from Series.idxmin/idxmax
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

It seems misleading to have the `out` parameter in the function signature when it is not used:

```
pd.Series.argmin(self, axis=None, out=None, skipna=True)
pd.Series.argmax(self, axis=None, out=None, skipna=True)
```

(unlike the `numpy.argmin/argmax` methods where `out` can be used)
