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: I77853c2a1b3a12b62db6100c8537ae35cf94b99e

tox.ini

@@ -51,8 +51,6 @@ commands =
   bandit -r barbican -x tests -n5 -s B105
 
 [testenv:genconfig]
-allowlist_externals = bash
-envdir = {toxworkdir}/pep8
 commands =
   oslo-config-generator --config-file etc/oslo-config-generator/barbican.conf
 
@@ -168,7 +166,6 @@ deps = bindep
 commands = bindep test
 
 [testenv:genpolicy]
-envdir = {toxworkdir}/pep8
 commands = oslopolicy-sample-generator --config-file=etc/oslo-config-generator/policy.conf
 
 [flake8:local-plugins]