[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)
I haven't tried this myself but I suspect del on an inherited attribute doesn't work (or at least doesn't fit into my understanding of Python's attribute representation). The closest I can get is:

```python
    @property
    def fit_predict(self):
        raise AttributeError
```
which could be included in the mixin, as long as we check the MRO resolves correctly.