Remove command-line entry point and simplify pyproject.toml

[adamltyson]

There were many napari-related entry points in the pyproject.toml. I think these aren't needed, so I've streamlined them.

[alessandrofelder]

I agree with removing the "napari.plugin" part, I think, and the renaming.

I am slightly unsure about removing project.scripts... because IIUC this removes a command line entry point that opens napari with the brainglobe-segmentation widget. AFAICT this CLI tool is undocumented (and hopefully not widely used), so we can remove it, but we should mention in this PR that we do so (which I have hereby done), and I wanted to double-check this removal was intended?

If this was intended, should we also remove the main function from segment.py as it's now unused, or document that your can do python segment.py to spin up napari with bg-segmentation?

[adamltyson]

I forgot we had that CLI entry point, it's from the dark days before napari had plugins! I've removed it and amended the PR title.

[adamltyson]

I've re-requested your review, just in case I've done something daft.