Removed was-open check for ComboBox to fix freeze on macOS

[cansik]

As already described in #6024, on macOS the combobox was freezing when it was opened.

• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves Combobox widget freezes when changing selections under MacOS #6024

Motivation and Context

At the moment it is not possible to select an item from a Combobox on macOS. This does not work in the Open3D App and also any other app using the Open3D GUI library.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

It seems that the problem is the following: On macOS the ImGUI method ImGui::IsPopupOpen always returns false and so does leads to a return of of Widget::DrawResult::REDRAW which causes the GUI to redraw itself, which will go on for ever.

To be honest, I am not quite sure why this check is done, maybe to change to combobox content if it has been changed while the user has the dropdown open (@errissa)?

Since it worked in version 0.15.0 I first thought it is an ImGUI problem, but the ImGUI version did not change from 0.15.0 to 0.16.0. And even with a more recent version (v1.88) it does not work. Because this bug has a big impact to any GUI (you can not even fully use the open3d app on macOS), I decided to fix it directly in Open3d. Maybe it makes sense to open an issue at ImGUI so that it will be fixed in a future release.

To fix the issue in Open3D, I have removed this check and simplified to redrawing to just redraw, when the selection has changed.

---

This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[errissa]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @cansik)

[ssheorey]

Thanks @cansik !