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: Ibbb4ab56ca84f5a8f9fe309a46469d49ff4ef9f7

tox.ini

@@ -36,7 +36,6 @@ commands =
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 
 [testenv:debug]
-envdir = {toxworkdir}/shared
 commands = oslo_debug_helper -t neutron/tests {posargs}
 
 [testenv:common]
@@ -109,12 +108,10 @@ commands =
   stestr run --slowest --combine --concurrency 1 neutron.tests.fullstack.test_securitygroup.TestSecurityGroupsSameNetwork.test_securitygroup {posargs}
 
 [testenv:releasenotes]
-envdir = {toxworkdir}/docs
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:pep8]
-envdir = {toxworkdir}/lint
 deps =
   {[testenv]deps}
   bashate>=2.1.1 # Apache-2.0
@@ -136,7 +133,6 @@ commands=
 allowlist_externals = bash
 
 [testenv:cover]
-envdir = {toxworkdir}/shared
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source neutron --parallel-mode
@@ -151,7 +147,6 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-envdir = {toxworkdir}/docs
 # Do not remove requirements.txt from deps list as without it
 # upper constraints will not be used for deps listed in requirements.txt
 # and may cause issues
@@ -162,7 +157,6 @@ deps =
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   make
@@ -171,7 +165,6 @@ commands =
   make -C doc/build/pdf
 
 [testenv:linkcheck]
-envdir = {toxworkdir}/docs
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b linkcheck doc/source doc/build/linkcheck
 
@@ -226,7 +219,6 @@ extension =
 import_exceptions = neutron._i18n
 
 [testenv:bandit]
-envdir = {toxworkdir}/lint
 deps = {[testenv:pep8]deps}
 # B104: Possible binding to all interfaces
 # B303: prohibit list calls: md5, sha1 for python<3.9
@@ -236,7 +228,6 @@ deps = {[testenv:pep8]deps}
 commands = bandit -r neutron -x tests -n5 -s B104,B303,B311,B324,B604
 
 [testenv:bashate]
-envdir = {toxworkdir}/lint
 deps = {[testenv:pep8]deps}
 commands = bash -c "find {toxinidir}             \
          -not \( -type d -name .tox\* -prune \)  \
@@ -250,11 +241,9 @@ commands = bash -c "find {toxinidir}             \
          -print0 | xargs -0 bashate -v -iE006 -eE005,E042,E043"
 
 [testenv:genconfig]
-envdir = {toxworkdir}/shared
 commands = bash {toxinidir}/tools/generate_config_file_samples.sh
 
 [testenv:genpolicy]
-envdir = {toxworkdir}/shared
 commands = oslopolicy-sample-generator --config-file=etc/oslo-policy-generator/policy.conf
 
 # This environment can be used to quickly validate that all needed system