Remove remains of jupyter bundlerextension

[m-rossi]

If you install or update jupyter_server after notebook the executable jupyter-bundlerextension will be overwritten because of the entrypoint defined in setup.py and the conda-forge-recipe. You will get the following error if you execute the following example command jupyter bundlerextension list

Traceback (most recent call last):
  File "/Users/USER/opt/anaconda3/bin/jupyter-bundlerextension", line 7, in <module>
    from jupyter_server.bundlerextensions import main
ModuleNotFoundError: No module named 'jupyter_server.bundlerextensions'

This can be fixed by reinstalling notebook with conda install notebook --force-reinstall, however this PR removes the remaining parts (left of #47) of the bundler from the codebase.

[Zsailer]

👍 This is fine for now.

I think we're going to keep the bundler in Jupyter Server (though there hasn't been any discussion around it, yet). That said, we'll need to rename the entrypoint and avoid colliding with notebook for a little while. If we do keep the bundler app in Jupyter Server, Notebook will lose the bundlerextensions entrypoint in favor of JS—or it at least it be shimmed to Jupyter Server;s bundler app.

In the meantime, removing the entrypoint here is fine. We'll need to create a new entrypoint in a future PR.

[Zsailer]

Thanks, @m-rossi! Merging.