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: I18ad8e5de031bc6be1b8e01860585ffae5ec0aa5

tox.ini

@@ -25,14 +25,12 @@ commands = flake8 {posargs}
 commands = {posargs}
 
 [testenv:functional]
-envdir = {toxworkdir}/functional
 setenv =
   {[testenv]setenv}
 commands =
   stestr run --exclude-regex ".tests.unit" {posargs}
 
 [testenv:cover]
-envdir = {toxworkdir}/shared
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source ovn_bgp_agent --parallel-mode
@@ -44,22 +42,18 @@ commands =
     coverage report --fail-under=82 --skip-covered
 
 [testenv:docs]
-envdir = {toxworkdir}/docs
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [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:debug]
-envdir = {toxworkdir}/shared
 commands = oslo_debug_helper {posargs}
 
 [testenv:genconfig]
-envdir = {toxworkdir}/shared
 commands =
     oslo-config-generator --config-file=etc/oslo-config-generator/bgp-agent.conf