Fix getting position of wxPopupWindow in wxMSW.

Don't offset the returned values by the parent window origin unnecessarily: as
popup windows are created as children of the desktop window in wxMSW, their
coordinates are already expressed in screen coordinate system (which is
exactly the same as display window coordinate system) and we must not try to
translate them to it once again, this is completely wrong and was probably a
left over from earlier wxPopupWindow implementation in which it wasn't created
as a child of desktop.

In 3.0 branch we still need to continue overriding virtual DoGetPosition() for
ABI reasons, but don't do anything in it any more.

Closes #4440.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76674 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775