remove jupyter-server-proxy due to lack of support for jupyterlab

should be available soon:
https://github.com/jupyterhub/jupyter-server-proxy/issues/153
but is not right now. In any case it does not seem that many people
are using the server proxy.
2 jobs for jbweston-master-patch-09884 in 21 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Mirror
passed #82514
allowed to fail
mirror repository

00:00:23

 
  Build
failed #82515
build image

00:21:26

 
Name Stage Failure
failed
build image Build
Conflicting Dependencies:
JupyterLab Extension Package
>=1.2.0 <1.3.0 >=0.16.0 <0.20.0@jupyterlab/application
>=1.2.0 <1.3.0 >=0.16.0 <0.20.0@jupyterlab/apputils
>=1.2.0 <1.3.0 >=0.16.0 <0.20.0@jupyterlab/notebook
See the log file for details: /tmp/jupyterlab-debug-v6zjy8lt.log
error building image: error building stage: waiting for process to exit: exit status 1
Authenticating with credentials from /root/.docker/config.json
ERROR: Job failed: exit code 1