Remove system_error rule in default constraint template

[binbin-li]

We have a rule that checks system_error from external data provider in the default given constraint template:

        general_violation[{"result": result}] {
          err := remote_data.system_error
          err != ""
          result := sprintf("System error calling external data provider: %s", [err])
        }

And Gatekeeper is defaulting to using failurePolicy: Ignore for admission request webhook errors. But the fail-open only applies to the webhook not external data provider. If the Ratify pod is down or unreachable, the current template will fail the validation which contradicts the fail-open policy. Wonder if we should remove the system_error rule so that the behavior could be consistent.