docs: kdoc: Remove a Python 2 comment

We no longer support Python 2 in the docs build chain at all, so we
certainly do not need to admonish folks to keep this file working with it.

Cc: Jani Nikula <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Acked-by: Jani Nikula <[e-mail]>
Tested-by: Akira Yokosawa <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/sphinx/kerneldoc.py

···
 # Authors:
 #    Jani Nikula <[e-mail]>
 #
-# Please make sure this works on both python2 and python3.
-#
 
 import codecs
 import os