fix: Remove JupyterScatter.hovering_widget

[manzt]

Write one to two sentences summarizing this PR

Removes JupyterScatter.hovering_widget. The JupyterScatter.hovered_point
property is not defined anywhere on the widget. I believe this traitlet was
likely added at some time but was removed, and this bug hasn't popped up due to
lack of accessing this field from others.

A recent release of anywidget inspects the fields on the widget class within
__init__, which invokes the @property decorator on the hovering_widget.
This led to an attribute error.

Description

What was changed in this pull request?

Why is it necessary?

Fixes #115

Checklist

• Provided a concise title as a semantic commit message (e.g. "fix: correctly handle undefined properties")
• CHANGELOG.md updated
• Tests added or updated
• Documentation in API.md/README.md added or updated
• Example(s) added or updated
• Screenshot, gif, or video attached for visual changes

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