Remove PYTHONHASHSEED=0 from tox pep8 job

Tempest has moved to new oslo-config-generator and this bug
workaround is no longer needed.

Change-Id: I50ffdcbc8a8f4830ba9c274af4c908e96467dad5
Closes-Bug: 1365136
diff --git a/tox.ini b/tox.ini
index c07dbbf..e575b4f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -98,7 +98,6 @@
 commands = python setup.py build_sphinx {posargs}
 
 [testenv:pep8]
-setenv = PYTHONHASHSEED=0
 commands =
    flake8 {posargs}
    {toxinidir}/tools/config/check_uptodate.sh