showbase: Clean up internal event handlers on destroy()

Fixes regression in cd6ad8285e11f780eee71cdb6b833b21b84d1d1b

direct/src/showbase/ShowBase.py

@@ -691,6 +691,7 @@ class ShowBase(DirectObject.DirectObject):
             allowAccessibilityShortcutKeys(True)
             self.__disabledStickyKeys = False
 
+        self.__directObject.ignoreAll()
         self.ignoreAll()
         self.shutdown()