Drop redundant injection of VIRTUAL_ENV

tox automatically injects the environment so no manual override is
necessary.

Change-Id: Id63e168f713416539bea4b357ad0ab535fdb49ba

tox.ini

@@ -4,23 +4,22 @@ envlist = py3,pep8
 
 [testenv]
 setenv =
-    VIRTUAL_ENV={envdir}
-    BRANCH_NAME=master
-    CLIENT_NAME=oslo.service
+  BRANCH_NAME=master
+  CLIENT_NAME=oslo.service
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 allowlist_externals = find
 commands =
-    find . -type f -name "*.py[co]" -delete
-    stestr run --slowest {posargs}
+  find . -type f -name "*.py[co]" -delete
+  stestr run --slowest {posargs}
 
 [testenv:pep8]
 deps =
-    pre-commit
+  pre-commit
 commands =
-    pre-commit run -a
+  pre-commit run -a
 
 [testenv:venv]
 commands = {posargs}