Use wxDELETE() and wxDELETEA() when possible.

Use wxDELETE[A]() functions which automatically NULL out their arguments after
deleting them instead of doing it manually.

Closes #9685.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64656 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/widgets/notebook.cpp

@@ -335,11 +335,7 @@ void BookWidgetsPage::CreateImageList()
     }
     else // no images
     {
-        if ( m_imageList )
-        {
-            delete m_imageList;
-            m_imageList = NULL;
-        }
+        wxDELETE(m_imageList);
     }
 
     // because of the bug in wxMSW we can't use SetImageList(NULL) - although