Remove installation of unnecessary pandoc and tex OS packages

[romainx]

Hello,

I've noticed that pandoc was installed 2 times. It's installed in the minimal-notebook as a conda package, then in the minimal-notebook as an OS package.

# The pandoc used is the one installed by conda
$ which pandoc

# /opt/conda/bin/pandoc

$ find / -name pandoc

# /usr/share/lintian/overrides/pandoc
# /usr/share/doc/pandoc
# /usr/share/pandoc
# /usr/bin/pandoc
# /opt/conda/bin/pandoc

$ ls -alsh  /opt/conda/bin/pandoc /usr/bin/pandoc

# 59M -rwxrwxr-x 1 jovyan users 59M Jan  6 00:01 /opt/conda/bin/pandoc
# 50M -rwxr-xr-x 1 root   root  50M Jan 26  2018 /usr/bin/pandoc

So I've removed the OS package installation from the minimal-notebook and written a test on the base-notebook to check that pandoc is working without this package. I've also performed a manual export to PDF test in a minimal-notebook.

It will contribute to slightly reduce the size of this image (same effect on downstream images as well).

Here is comparison

REPOSITORY                 TAG                 IMAGE ID            CREATED             SIZE
jupyter/minimal-notebook   latest              a817cb6de443        37 seconds ago      2.49GB
jupyter/minimal-notebook   <none>              774718b3f9c5        2 weeks ago         2.54GB

Best

[parente]

I don't see pandoc installed elsewhere in minimal-notebook via conda. I'm guessing one of the conda packages in base-notebook now specifies pandoc as a dependency and so it's getting installed there. I don't see it listed in the recipes for notebook, jupyterlab, or jupyterhub on conda-forge, so I'm a bit confused about where it's getting pulled in. We should probably make it an explicit conda install until we know for sure it's not an accidental dependency.

Do we know if tex still needs to be installed at the OS level or if the pandoc conda package installs everything it needs too?

[parente]

@romainx I'm cool with merging this PR if you are. You had it marked WIP. Are you planning any other changes or should I ship it?

[romainx]

Hey @parente. No sorry I just did not change the name, I will not perform other change on this PR. So let's go!
Thank you.