Drop `nbdime` diffs

[jtpio]

Currently the extension exposes two commands for adding cell diffs:

jupyterlab-cell-diff/src/plugin.ts

Line 76 in b814678

commands.addCommand('jupyterlab-cell-diff:show-codemirror', {

jupyterlab-cell-diff/src/plugin.ts

Line 182 in b814678

commands.addCommand('jupyterlab-cell-diff:show-nbdime', {

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.