DOC: Remove mention of defunct viz packages

[pllim]

Stumbled upon this and figured this should be removed.

[dhomeier]

Wondering if there should not be a reference to Jdaviz somewhere instead – not strictly a plugin, but that would be the currently recommended solution for previous use of those plugins, right?

[pllim]

Jdaviz is a standalone package using the Jupyter stuff, so I don't think we should mention it on this particular page.

[dhomeier]

So for the standalone app those packages are gone without a replacement (I don't know if the archived versions would still be functional)?

[pllim]

Yes, they gone. No more supported.