Drop notebook 6

[marthacryan]

It appears that the entire packages/javascript/jupyterlab_plotly directory contains code that is either used to support very old versions of jupyter lab/notebook/voila, or is not used at all. This PR removes the entire directory in favor of having a very small packages/python/plotly/js directory that contains only the widget code (see #4663 for adding the widget code into that directory).

The last time that the jupyter extension was significantly changed was 3 years ago in #3142. @fcollonval described the files in the packages/javascript/jupyterlab_plotly directory in this comment. He did add support for federated extensions (new jupyter lab/notebook build), but I wasn't able to find evidence that having an extension installed in jupyterlab >=3 or jupyter notebook >= 7 is even necessary for plotly. @fcollonval if you know of any reason that that isn't true, please let me know!

To test:

• Checkout branch
• Create a new python env (conda, mamba, pipenv, etc)
• git clean -f -x -d
• cd packages/python/plotly
• pip install -e .
• pip install jupyter
• Try it out in jupyter lab: jupyter lab
• Try it out in jupyter notebook: jupyter notebook