Remove unnecessary line in test_auth

TestCase sets up the kvs identity backend already, by using
test_overrides.conf, so this line doesn't do anything and is going
to lead to confusion if test_overrides.conf changes to a different
backend.

Change-Id: I3f62e70a0dd7a27b675a29a56bfbccffa1b9be50
Merge "Remove unnecessary line in test_auth"