README: remove python3-distutils dep for ubuntu

distutils has been deprecated in Python 3.10 [1] and was removed in
Python 3.12.

The functionality of distutils has already been completely replaced
by tools such as setuptools.

Tested: Build an image on Ubuntu 22.04, which uses Python 3.10, without
        python3-distutils being installed.

[1] https://docs.python.org/3.10/whatsnew/3.10.html#distutils-deprecated



Change-Id: I5dd433df58512eb01f3ee88f137b0ed108024d03
Signed-off-by: Tan Siewert <tan@siewert.io>