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: Ia842a72d618cf4b27ebaab0f2c576bb7eb76ab24

diskimage-create/tox.ini

@@ -4,7 +4,6 @@ envlist = bashate,docs,build,test
 
 [testenv]
 basepython = python3
-envdir = {toxworkdir}/venv
 setenv = VIRTUAL_ENV={envdir}
 passenv =
     http_proxy