Remove dummy front-end nbextension

[akaihola]

The extension currently includes a dummy jupyter_abc/static/index.js file which defines a front-end extension and can be installed and enabled using jupyter nbextension install and jupyter nbextension enable.

The front-end part of the extension should probably be removed unless

• someone comes up with features which are best implemented there
• existing JavaScript code injected by extension initialization and the %%abc magic command would be better moved into the front-end part (Jupyter expert opinions appreciated here)