python: Remove ui.GUI begin and end

[Xuanwo]

• Previous discussion: https://github.com/taichi-dev/taichi/pull/3055#discussion_r719066130
• Related issue: https://github.com/taichi-dev/taichi/issues/3015

In PR #3055, we added GUI.sub_window.

So it's possible to use sub_window to replace all usage about GUI.begin and GUI.end.

This will make the API cleaner and easier for users, and help users prevent mismatched begins and ends.

[AmesingFlank]

Thanks! Please remember to also update the docs and the examples in examples/ggui_examples when you make this change.

[Xuanwo]

Is this issue still valid?