scrolledwindow: Stop using scroll cursors

scrolledwindow: Stop using scroll cursors

We used to override cursor to use all-scroll while the
content is being scrolled. Unfortunately, there is several
problems with this:
- It is really only expected certain devices, and we don't
  have the device information on Wayland
- With the way cursor setting works in GTK4, non-NULL cursors
  of the content (eg the text views ibeam) win, making the
  scroll cursor not show up
- Under X11, we seem to miss scroll end events and then
  the scroll cursor gets stuck
Therefore, just remove this feature.