Remove dask xgboost

Fixes #911.

This package only works with xgboost <= 0.90: https://github.com/dask/dask-xgboost/blob/master/requirements.txt#L1

BUG=175051617

Author: rosbo

Dockerfile

@@ -229,7 +229,6 @@ RUN pip install mpld3 && \
     pip install geoplot && \
     pip install eli5 && \
     pip install implicit && \
-    pip install dask-ml[xgboost] && \
     pip install kaggle && \
     /tmp/clean-layer.sh