Don't verify existence of autorenew poll messages

There are several situations in which it is a valid system state for
Keys to PollMessages on Domains to point to entities that don't exist,
chief amongst them when a domain has been soft-deleted.  We also can't
simply null this out when soft-deleting domains, because there may be
an un-acked recurrence meaning it can't be deleted.

Given this, it doesn't make sense to check for entity existence here,
as it is not a system invariant and cannot easily be made to be so.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122300368

java/google/registry/monitoring/whitebox/VerifyEntityIntegrityAction.java

@@ -274,7 +274,6 @@ public class VerifyEntityIntegrityAction implements Runnable {
           DomainResource domain = (DomainResource) domainBase;
           verifyExistence(key, domain.getApplication());
           verifyExistence(key, domain.getAutorenewBillingEvent());
-          verifyExistence(key, domain.getAutorenewPollMessage());
           for (GracePeriod gracePeriod : domain.getGracePeriods()) {
             verifyExistence(key, gracePeriod.getOneTimeBillingEvent());
             verifyExistence(key, gracePeriod.getRecurringBillingEvent());