Remove evt.preventDefault() calls on dropdown mouse events.

The evt.preventDefault() on mousedown breaks ComboBox on iOS, by cancelling the mouseup
and click events, thus preventing selection of an option.

According to the comment, the evt.preventDefault() call on mousedown was to
"prevent focus shift on list scrollbar press".
But I still lost focus when clicking the scrollbar, at least on IE8.
Plus which, even if clicking the scrollbar loses focus, it doesn't seem like a problem.

There were also evt.preventDefault() calls on the other events (mouseup, mouseover, etc.) but I
don't see any reason for them.

Fixes #17752 on trunk.