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: I060355cf8d8fb76d3172d977c3aa17e28dceee1d

Author: kajinamit

tox.ini

@@ -31,7 +31,6 @@ commands =
   sphinx-build -a -E -W -j auto -d doc/build/doctrees -b html doc/source doc/build/html
 
 [testenv:docs-starlingxdocs]
-envdir = {toxworkdir}/docs
 commands =
   rm -rf doc/build/html doc/build/doctrees
   sphinx-build -a -E -W -j auto -d doc/build/doctrees -b html -D html_theme=starlingxdocs doc/source doc/build/html
@@ -63,14 +62,12 @@ commands =
   sphinx-build -a -E -W -j auto -d api-ref/build/doctrees -b html -D html_theme=starlingxdocs api-ref/source api-ref/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 commands =
   rm -rf doc/build/pdf
   sphinx-build -a -E -W -j auto -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
 
 [testenv:pdf-docs-starlingxdocs]
-envdir = {toxworkdir}/docs
 commands =
   rm -rf doc/build/pdf
   sphinx-build -a -E -W -j auto -b latex -D html_theme=starlingxdocs doc/source doc/build/pdf