Fix to remove call to registered_names which no longer exists in bokeh

[alexmilowski]

I'm really sure this is the correct change. It appears the IPyWidget is available like other things via Bokeh.Panel.IPyWidget

The issue is the use of registered_names generates a lot of JavaScript exceptions when we use things like the Tabulator.

[philippjfr]

Thanks for the fix. Have to think about this a little more. To give a bit of background, Panel has two approaches for rendering IPyWidgets:

1. When you're inside a notebook environment it uses the notebooks WidgetManager and will render itself using the Panel ipywidget model.
2. When running on a server or when embedded in a website it instead uses the ipywidgets_bokeh implementation of a renderer.

So really this code should only check for the ipywidgets_bokeh model if we are in scenario 2.

[codecov]

Codecov Report

Merging #4939 (b838b3e) into main (0bc6499) will increase coverage by 10.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main    #4939       +/-   ##
===========================================
+ Coverage   73.38%   83.43%   +10.05%     
===========================================
  Files         270      270               
  Lines       38218    38221        +3     
===========================================
+ Hits        28046    31891     +3845     
+ Misses      10172     6330     -3842     

Flag	Coverage Δ
ui-tests	40.88% <33.33%> (?)
unitexamples-tests	73.38% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
panel/io/notebook.py	44.44% <100.00%> (+0.64%)	⬆️

... and 57 files with indirect coverage changes

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