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: Id7d7ebd5487fbd261433fa21bb86b8b3c2d6b9de

Author: kajinamit

tox.ini

@@ -7,7 +7,6 @@ ignore_basepython_conflict = True
 [testenv]
 basepython=python3
 usedevelop = True
-setenv = VIRTUAL_ENV={envdir}
 deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
 allowlist_externals = find
@@ -28,7 +27,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   rm