[jupyterhub] don't install kernel to user

jupyterhub/base/Dockerfile

@@ -69,7 +69,7 @@ RUN mkdir -p /usr/local/venv && \
         sklearn \
         'mxnet-cu101>=1.6.0,<1.7.0' \
         && \
-    ipython kernel install --user --name=default
+    ipython kernel install --name=default
 
 COPY jupyter_notebook_config.py /etc/jupyter/
 ENV JUPYTER_CONFIG_DIR=/etc/jupyter