Instead of just deleting m_popupInterface, call Destroy() of its wxWindow-based popup...

Instead of just deleting m_popupInterface, call Destroy() of its wxWindow-based popup control obtained via GetControl() member function. Also still delete m_popupInterface if there was no popup control, or if it was implemented as a separate class (versus being multiple-inherited along side wxComboPopup, as has been the common practice).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67320 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775