Don't try to determine the clicked item ourselves in wxMSW wxListBox.

Don't try to determine the clicked item ourselves in wxMSW wxListBox.

This doesn't work when the listbox is scrolled as the result of a click to
make the selected item fully visible and results in the index of the item
being off by 1 in the generated event which is a pretty serious problem.

Fix it by simply retrieving the item from the listbox itself, without doing
any hit testing. This seems to give the correct result in all cases and also
makes the code much simpler as we don't have to use 2 different ways of
finding the item depending on whether it was selected using the keyboard or
the mouse and makes it unnecessary to keep track of how the selection was done
completely, i.e. reverts r64498 which is not needed any more.

Closes #14635.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72444 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775