tox: Drop envdir

tox now always recreates an env although the env is shared using envdir
options.

According to the maintainer of tox, this functionality is not intended
to be supported.
https://github.com/tox-dev/tox/issues/425#issuecomment-1011944293

Change-Id: Ie5e8b4f10a509d29333836b6836c6ae4ef48a2e6
1 file changed