Get rid of ReferenceUnions entirely

This is the third and final phase in the migration away from ReferenceUnions.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138778148

javatests/google/registry/model/ImmutableObjectTest.java

@@ -19,7 +19,6 @@ import static com.google.common.collect.Maps.newHashMap;
 import static com.google.common.collect.Sets.newHashSet;
 import static com.google.common.truth.Truth.assertThat;
 import static google.registry.model.ImmutableObject.cloneEmptyToNull;
-import static google.registry.testing.DatastoreHelper.persistActiveContact;
 import static google.registry.testing.DatastoreHelper.persistResource;
 import static google.registry.util.DateTimeUtils.START_OF_TIME;
 
@@ -31,7 +30,6 @@ import com.googlecode.objectify.Key;
 import com.googlecode.objectify.ObjectifyService;
 import com.googlecode.objectify.annotation.Entity;
 import com.googlecode.objectify.annotation.Id;
-import google.registry.model.domain.ReferenceUnion;
 import google.registry.testing.AppEngineRule;
 import google.registry.util.CidrAddressBlock;
 import java.util.ArrayDeque;
@@ -255,6 +253,7 @@ public class ImmutableObjectTest {
 
   /** Subclass of ImmutableObject with keys to other objects. */
   public static class RootObject extends ImmutableObject {
+
     Key<ValueObject> hydrateMe;
 
     @DoNotHydrate
@@ -263,8 +262,6 @@ public class ImmutableObjectTest {
     Map<String, Key<ValueObject>> map;
 
     Set<Key<ValueObject>> set;
-
-    ReferenceUnion<?> referenceUnion;
   }
 
   /** Simple subclass of ImmutableObject. */
@@ -308,11 +305,4 @@ public class ImmutableObjectTest {
     root.set = ImmutableSet.of(Key.create(persistResource(ValueObject.create(1, "foo"))));
     assertThat(root.toHydratedString()).contains("foo");
   }
-
-  @Test
-  public void testToHydratedString_expandsReferenceUnions() {
-    RootObject root = new RootObject();
-    root.referenceUnion = ReferenceUnion.create(Key.create(persistActiveContact("foo")));
-    assertThat(root.toHydratedString()).contains("foo");
-  }
 }