[cmake] Do not install jupyter into the virtualenv

If you need this you can still do it manually, but there is absolutely
no reason to have it unconditionally.

python/CMakeLists.txt

-# Install some additional packages into the virtualenv.
-#
-# Install path.py in version 7.7 is to avoid a bug in
-# IPythons dependency pickleshare.
-dune_execute_process(COMMAND ${DUNE_PYTHON_VIRTUALENV_EXECUTABLE} -m pip install path.py==7.7 tornado>=4.0 ipython>=4.0.0 jupyter>=1.0.0
-                     ERROR_MESSAGE "Error installing jupyter in virtualenv")
-
 # Install all the external packages that we have as submodules
 dune_python_install_package(PATH pytools)
 dune_python_install_package(PATH cgen)