Remove leftover reference to juypterhub

repo2docker/detectors.py

@@ -733,9 +733,7 @@ class LegacyBinderDockerBuildPack(DockerBuildPack):
     USER main
     WORKDIR /home/main/notebooks
     ENV PATH /home/main/anaconda2/envs/python3/bin:$PATH
-    ARG JUPYTERHUB_VERSION
     RUN conda install -yq -n python3 notebook==5.0.0 ipykernel==4.6.0 && \
-        pip install --no-cache-dir jupyterhub==${JUPYTERHUB_VERSION} && \
         conda remove -yq -n python3 nb_conda_kernels && \
         conda install -yq -n root ipykernel==4.6.0 && \
         /home/main/anaconda2/envs/python3/bin/ipython kernel install --sys-prefix && \