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: Ia13daf5d17186f3208391c4d37e0e0c774cf30ab

tox.ini

@@ -33,7 +33,6 @@ commands =
   bandit -r keystone -x 'keystone/tests/*'
 
 [testenv:fast8]
-envdir = {toxworkdir}/pep8
 deps = {[testenv:pep8]deps}
 commands =
   {toxinidir}/tools/fast8.sh
@@ -125,7 +124,6 @@ commands=
 # the workaround after this patch has merged:
 # https://review.opendev.org/#/c/679860
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   make
@@ -138,12 +136,10 @@ commands =
   make -C doc/build/pdf
 
 [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
 
 [testenv:api-ref]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands =
   bash -c "rm -rf api-ref/build"