Remove tox from python-tools-install.

Remove tox from python-tools-install.

If users need a system pip install of tox
they can include tox-install.
However, unless they need pyenv
I would suggest they use lf-env.

Order of system pip install of tox matters,
whichever one is installed most recently
is the one that is going to be called.
removing tox from python tools install
allows the install of tox in tox-install
to not be skipped, and in this way
we can ensure that the py3 version
is the once most recently installed

Issue-Id: RELENG-3189
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
Change-Id: Ie904f42821f6598a389854e8876448237cfac3dd