Pop event handler from wxDVC edit control before destroying it.

This fixes the assert in wxWindow dtor checking that all event handlers pushed
on the window were removed.

Also fix a memory leak by not creating an unnecessary object of wxKillRef
class. In fact remove this class entirely.

Closes #11732.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64216 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dvrenderers.h

@@ -154,6 +154,9 @@ public:
     wxControl *GetEditorCtrl() { return m_editorCtrl; }
 
 protected:
+    // Called from {Cancel,Finish}Editing() to cleanup m_editorCtrl
+    void DestroyEditControl();
+
     wxString                m_variantType;
     wxDataViewColumn       *m_owner;
     wxWeakRef<wxControl>    m_editorCtrl;