remove DNS resources to fix SSL test

[modular-magician]

simple fix for the failing TestAccComputeManagedSslCertificate_managedSslCertificateBasicExample test. DNS records are not needed to create an SSL cert or to associate it with a proxy/URL map.

Yes, technically the cert will never actually be live due to the failed DNS check. But we're deleting the cert object anyways way before any validation can be completely successfully.

fixes {hashicorp/terraform-provider-google#12528 }

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Derived from GoogleCloudPlatform/magic-modules#6726