Remove duplicate version of lookupSuperordinateDomain() function

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148657943

java/google/registry/flows/host/HostCreateFlow.java

@@ -96,8 +96,8 @@ public final class HostCreateFlow implements TransactionalFlow {
     // The superordinate domain of the host object if creating an in-bailiwick host, or null if
     // creating an external host. This is looked up before we actually create the Host object so
     // we can detect error conditions earlier.
-    Optional<DomainResource> superordinateDomain = Optional.fromNullable(
-        lookupSuperordinateDomain(validateHostName(targetId), now));
+    Optional<DomainResource> superordinateDomain =
+        lookupSuperordinateDomain(validateHostName(targetId), now);
     verifyDomainIsSameRegistrar(superordinateDomain.orNull(), clientId);
     boolean willBeSubordinate = superordinateDomain.isPresent();
     boolean hasIpAddresses = !isNullOrEmpty(command.getInetAddresses());

java/google/registry/flows/host/HostFlowUtils.java

@@ -86,12 +86,12 @@ public class HostFlowUtils {
   }
 
   /** Return the {@link DomainResource} this host is subordinate to, or null for external hosts. */
-  static DomainResource lookupSuperordinateDomain(
+  public static Optional<DomainResource> lookupSuperordinateDomain(
       InternetDomainName hostName, DateTime now) throws EppException {
     Optional<InternetDomainName> tld = findTldForName(hostName);
     if (!tld.isPresent()) {
       // This is an host on a TLD we don't run, therefore obviously external, so we are done.
-      return null;
+      return Optional.absent();
     }
     // This is a subordinate host
     String domainName = Joiner.on('.').join(Iterables.skip(
@@ -100,7 +100,7 @@ public class HostFlowUtils {
     if (superordinateDomain == null || !isActive(superordinateDomain, now)) {
       throw new SuperordinateDomainDoesNotExistException(domainName);
     }
-    return superordinateDomain;
+    return Optional.of(superordinateDomain);
   }
 
   /** Superordinate domain for this hostname does not exist. */

java/google/registry/flows/host/HostUpdateFlow.java

@@ -130,7 +130,7 @@ public final class HostUpdateFlow implements TransactionalFlow {
     String newHostName = firstNonNull(suppliedNewHostName, oldHostName);
     // Note that lookupSuperordinateDomain calls cloneProjectedAtTime on the domain for us.
     Optional<DomainResource> newSuperordinateDomain =
-        Optional.fromNullable(lookupSuperordinateDomain(validateHostName(newHostName), now));
+        lookupSuperordinateDomain(validateHostName(newHostName), now);
     verifyUpdateAllowed(command, existingHost, newSuperordinateDomain.orNull());
     if (isHostRename && loadAndGetKey(HostResource.class, newHostName, now) != null) {
       throw new HostAlreadyExistsException(newHostName);