Remove widgetsnbextension and ipywidgets recipes

[martinRenou]

• Technically the widgetsnbextension recipe can be removed from here when Migration plan for noarch and removing the link scripts conda-forge/widgetsnbextension-feedstock#52 is fixed
• The ipywidgets recipe can be removed when it does not depend on ipykernel anymore, or when we have a proper ipykernel recipe for emscripten-32. The ipykernel dependency can be removed if Generic way for kernels to expose comms jupyter-xeus/xeus-python#342 is resolved. Removing the ipykernel dependency is a good approach I think, because it will benefit the classic xeus-python and pyolite.

[martinRenou]

See jupyter-widgets/ipywidgets#3533 for an approach to remove the ipykernel dependency in ipywidgets

[jtpio]

For reference the ipywidgets recipe was first removed in #298.

But then restored in #300 as the ipykernel dependency was added back upstream in ipywidgets for now. At least for 8.0.x it seems, might change with 8.x.

[jtpio]

Closing as fixed by #602, thanks!