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

tox.ini

@@ -6,7 +6,6 @@ envlist = py3,pep8
 basepython = python3
 usedevelop = True
 setenv =
-   VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_STDOUT_CAPTURE=1
    OS_STDERR_CAPTURE=1