remove surplus message

The log message "PaletteWindowWidget popup get_visible True" is not
useful, and is removed.  It can be made to occur in Terminal by repeated
right-click on any of the first three toolbar button.

src/sugar3/graphics/palettewindow.py

@@ -394,7 +394,6 @@ class _PaletteWindowWidget(Gtk.Window):
 
     def popup(self, invoker):
         if self.get_visible():
-            logging.error('PaletteWindowWidget popup get_visible True')
             return
         self.connect('enter-notify-event', self.__enter_notify_event_cb)
         self.connect('leave-notify-event', self.__leave_notify_event_cb)