remove unregister trait-change callback in ExtensionManager.__del__

Due to reference cycles, this can never be called at a time when it would matter,
but seems to cause weird crashes sometimes during interpreter teardown on Python 3.5 with traitlets 4.1.

IPython/core/extensions.py

@@ -59,11 +59,6 b' class ExtensionManager(Configurable):'
         )
         self.loaded = set()
 
-    def __del__(self):
-        self.shell.on_trait_change(
-            self._on_ipython_dir_changed, 'ipython_dir', remove=True
-        )
-
     @property
     def ipython_extension_dir(self):
         return os.path.join(self.shell.ipython_dir, u'extensions')