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: I2cef53e151b9da265989c55e165b7521af32d44d

tox.ini

@@ -77,7 +77,6 @@ setenv = PYTHONHASHSEED=0
 commands = {posargs:}
 
 [testenv:debug]
-envdir = {toxworkdir}/venv
 commands = oslo_debug_helper -t ironic_python_agent/tests/unit {posargs}
 
 [testenv:docs]
@@ -126,13 +125,11 @@ import_exceptions = ironic.openstack.common.gettextutils._,testtools.matchers
 
 [testenv:checkconfig]
 sitepackages = False
-envdir = {toxworkdir}/venv
 commands =
   {toxinidir}/tools/config/check_uptodate.sh
 
 [testenv:genconfig]
 sitepackages = False
-envdir = {toxworkdir}/venv
 allowlist_externals = mkdir
 commands =
   mkdir -p etc/ironic_python_agent