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/dns/RefreshDnsAction.java

@@ -70,7 +70,7 @@ public final class RefreshDnsAction implements Runnable {
   }
 
   private static void verifyHostIsSubordinate(HostResource host) {
-    if (host.getSuperordinateDomain() == null) {
+    if (!host.isSubordinate()) {
       throw new BadRequestException(
           String.format("%s isn't a subordinate hostname", host.getFullyQualifiedHostName()));
     }