ipatests: do not set dns_lookup to true
dns_lookup was set to false during ipa client installation which prevented
searches for SRV records for Kerberos servers.
Since https://pagure.io/freeipa/issue/6523 is fixed, dns_lookup is always True
now and the fixture is not needed anymore.
Reviewed-By: Francois Cami <fcami@redhat.com>