Remove "builtins = _" from tox.ini
Remove "builtins = _" from tox.ini

"_" (ugettext_lazy) is defined by usual imports,
so "builtins = _" in tox.ini is unnecessary.

Change-Id: Ie3452b1ae1b57e0742f1846ff24c8edbdb850df1

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard