tox: Drop envdir

tox now always recreates an env although the env is shared using envdir
options.
~~~
$ tox -e genpolicy
genpolicy: recreate env because env type changed from
{'name': 'genconfig', 'type': 'VirtualEnvRunner'} to
{'name': 'genpolicy', 'type': 'VirtualEnvRunner'}
~~~

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: I1c4a0780f890b679b83c7ff8d87c549cee3cdd58

tox.ini

@@ -7,8 +7,6 @@ usedevelop = True
 passenv = ZUUL_CACHE_DIR
           REQUIREMENTS_PIP_LOCATION
 install_command = pip install {opts} {packages}
-setenv =
-   VIRTUAL_ENV={envdir}
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -49,7 +47,6 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 basepython = python3
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   rm