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: I3155eb73edb09184a29522708ef2f676bd06fb7f

tox.ini

@@ -10,14 +10,12 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
    DEVSTACK_VENV={env:DEVSTACK_VENV}
-   VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
 [testenv:cover]
 setenv =
-    VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source cloudkittyclient --parallel-mode
 commands =
     stestr run {posargs}
@@ -80,7 +78,6 @@ deps =
 commands = sphinx-build --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   make