Drop `nbdime` diffs

[jtpio]

Currently the extension exposes two commands for adding cell diffs:

https://github.com/jupyter-ai-contrib/jupyterlab-cell-diff/blob/b814678435ade179e630bfc12c3faa744d721c05/src/plugin.ts#L76

https://github.com/jupyter-ai-contrib/jupyterlab-cell-diff/blob/b814678435ade179e630bfc12c3faa744d721c05/src/plugin.ts#L182

However the nbdime command requires having nbdime installed on the server.

Dropping support for this command would help simplify the extension, which would become pure frontend.