Remove "builtins = _" from tox.ini

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

Change-Id: I2d429f5a4783f8da44739da7fe3ba1d6c76f4146
Merge "Remove "builtins = _" from tox.ini"