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.
tox-dev/tox#425 (comment)

Change-Id: Ibbd2be42a566374e86ad57fc6ae1202bae6d7d43

Author: kajinamit

tox.ini

@@ -46,7 +46,6 @@ commands =
 [testenv:docs]
 setenv = PYTHONHASHSEED=0
 sitepackages = False
-envdir = {toxworkdir}/venv
 deps =
     -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
@@ -58,7 +57,6 @@ commands =
 allowlist_externals = make
 setenv = PYTHONHASHSEED=0
 sitepackages = False
-envdir = {toxworkdir}/venv
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -b latex doc/source doc/build/pdf