Replace 'host.getSubordinateHost() != null' with 'host.isSubordinate()'

This is a cleanup in preparation for the next change that does a lot
of work with subordinate hosts, to make it easier to reason about in
complex code.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146689904

java/google/registry/batch/DeleteContactsAndHostsAction.java

@@ -367,7 +367,7 @@ public class DeleteContactsAndHostsAction implements Runnable {
             historyEntryForDelete);
       } else if (existingResource instanceof HostResource) {
         HostResource host = (HostResource) existingResource;
-        if (host.getSuperordinateDomain() != null) {
+        if (host.isSubordinate()) {
           dnsQueue.addHostRefreshTask(host.getFullyQualifiedHostName());
           ofy().save().entity(
               ofy().load().key(host.getSuperordinateDomain()).now().asBuilder()