[MRG+1] Fix FeatureAgglomeration docs

The docs are not displayed properly here, http://scikit-learn.org/stable/modules/generated/sklearn.cluster.FeatureAgglomeration.html (I keep running into these)
Wouldn't it be better to remove the method instead? Adding the following after the class definition should work.

```python
del FeatureAgglomeration.fit_predict
```

I agree this goes against the type semantics of Python subclassing but raising a `NotImplementedError` is not that much better in this regard.