Remove elementtree from installation instructions

This was only required for Python < 2.5, which will be dropped
in Supervisor 4.0.

Author: mnaberez

docs/installing.rst

@@ -67,9 +67,6 @@ which is internet-connected:
 - meld3 (latest) from `http://www.plope.com/software/meld3/
   <http://www.plope.com/software/meld3/>`_.
 
-- elementtree (latest) from `http://effbot.org/downloads#elementtree
-  <http://effbot.org/downloads#elementtree>`_.
-
 Copy these files to removable media and put them on the target
 machine.  Install each onto the target machine as per its
 instructions.  This typically just means unpacking each file and