Unset previously defined cd function rather than redefine it

It is cleaner to unset the previously defined cd function in
postdeactivate rather than redefine it since to avoids leaving an
unexpected function in the environment after a deactivate.

Change-Id: Iac40287ec5290f2f4523004a9d8f0b2a2b981f90

Author: lonetwin

docs/source/tips.rst

@@ -135,12 +135,9 @@ it a proper path then it will do the right thing.
 
 And to finally restore the default behaviour of ``cd`` once you
 bailout of a VENV via a ``deactivate`` command, you need to add this
-as a ``postdeactivate`` hook (`source 
-<http://github.com/kdeldycke/dotfiles/commit/80470b>`_)::
+as a ``postdeactivate`` hook::
 
-    cd () {
-        builtin cd "$@"
-    }
+    unset -f cd
 
 Clean up environments on exit
 =======================================