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: Iff90014cbe2d0c1550005cfb99a9b8fa7bc078bd

tox.ini

@@ -9,8 +9,8 @@ ignore_basepython_conflict = True
 [testenv]
 basepython = python3
 usedevelop = True
-setenv = VIRTUAL_ENV={envdir}
-         DJANGO_SETTINGS_MODULE=watcher_dashboard.test.settings
+setenv =
+  DJANGO_SETTINGS_MODULE=watcher_dashboard.test.settings
 # Note the hash seed is set to 0 until horizon can be tested with a
 # random hash seed successfully.
          PYTHONHASHSEED=0
@@ -55,7 +55,6 @@ deps =
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   rm