Remove getOnlyElement()

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124846747

java/google/registry/model/domain/DomainBase.java

@@ -205,7 +205,8 @@ public abstract class DomainBase extends EppResource {
     return FluentIterable
         .from(nullToEmpty(allContacts))
         .filter(IS_REGISTRANT)
-        .getOnlyElement()
+        .first()
+        .get()
         .getContactRef();
   }