[tox] remove DEVSTACK_VENV variable

This variable seems to be unused (grepping for it returns
only results from tox.ini, which are getting deleted here).

Looking at the PS where it got introduced
https://review.opendev.org/c/openstack/python-cloudkittyclient/+/893046
it seems like it was supposed to be used in the functional tests,
but there were issues and the path was hardcoded instead.

Change-Id: I8a75f5849894e5a0c327bd6e6b9ef87dd8f1f820
Signed-off-by: Jaromir Wysoglad <jwysogla@redhat.com>

tox.ini

@@ -7,8 +7,6 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 install_command = pip install -U {opts} {packages}
-setenv =
-   DEVSTACK_VENV={env:DEVSTACK_VENV}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
@@ -40,7 +38,6 @@ passenv =
       OS_AUTH_URL
       OS_USERNAME
       OS_ENDPOINT
-      DEVSTACK_VENV
       VIRTUAL_ENV
 setenv = OS_RATING_API_VERSION=1
 commands = stestr run --concurrency=1 --test-path ./cloudkittyclient/tests/functional/v1
@@ -59,7 +56,6 @@ passenv =
       OS_AUTH_URL
       OS_USERNAME
       OS_ENDPOINT
-      DEVSTACK_VENV
       VIRTUAL_ENV
 setenv = OS_RATING_API_VERSION=2
 commands = stestr run --concurrency=1 --test-path ./cloudkittyclient/tests/functional/v2