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: I16073e791defeb3bd454b1c9a4f533e66fe1a260

tox.ini

@@ -66,7 +66,6 @@ commands =
   stestr run {posargs}
 
 [testenv:releasenotes]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
@@ -117,7 +116,6 @@ deps = -c{env:TOX_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/ra
 commands = sphinx-build -W -a -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   make