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: I2f5ed23988842f41fe91a90a31041106100c9895

tox.ini

@@ -70,7 +70,6 @@ deps =
       -r{toxinidir}/requirements.txt
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   make