Remove unnecessary dynamic cast in wxComboPopupWindow.

Remove unnecessary dynamic cast in wxComboPopupWindow.

Objects of this type are either always of wxPopupTransientWindow type, in
which case just a static_cast<> is enough and we don't need a dynamic one, or
is never of its type in which case this code shouldn't be compiled and if it
is, then static_cast<> will correctly fail preventing it from compiling.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71622 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775