Some config options in pyproject.toml are unused #5537

The pyproject.toml currently contains several sections starting with tool.jupyter-packaging.* and tool.hatch.* which, as far as I understand, are unused since we are using setuptools as our build backend, not hatch or jupyter-packaging. And we also have no setup.py file, which would be the other place those config options might have an impact.

At the very least, we should remove those sections; plausibly we should also consider whether it's worth moving to hatchling as a build backend, which might let us automate some of the js bundling steps that we are currently doing semi-manually.

See also #5476 which put this on my radar.