Scope down TransferData to only ContactResource and DomainResource

HostResource and DomainApplication are not transferable, (or at
least, not directly in the case of hosts) and have no need for
the TransferData field. In a flat-flow world, we can push it down
to where it's actually used.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139201423

javatests/google/registry/model/host/HostResourceTest.java

@@ -149,14 +149,6 @@ public class HostResourceTest extends EntityTestCase {
         .isNotNull();
   }
 
-  @Test
-  public void testEmptyTransferDataBecomesNull() throws Exception {
-    HostResource withNull = new HostResource.Builder().setTransferData(null).build();
-    HostResource withEmpty = withNull.asBuilder().setTransferData(TransferData.EMPTY).build();
-    assertThat(withNull).isEqualTo(withEmpty);
-    assertThat(withEmpty.hasTransferData()).isFalse();
-  }
-
   @Test
   public void testImplicitStatusValues() {
     // OK is implicit if there's no other statuses.
@@ -205,7 +197,6 @@ public class HostResourceTest extends EntityTestCase {
         hostResource.asBuilder()
             .setLastSuperordinateChange(superordinateChangeTime)
             .setLastTransferTime(hostTransferTime)
-            .setTransferData(null)
             .build());
     return hostResource.cloneProjectedAtTime(clock.nowUtc()).getLastTransferTime();
   }
@@ -279,8 +270,6 @@ public class HostResourceTest extends EntityTestCase {
             .build())
         .build());
     HostResource afterTransfer = hostResource.cloneProjectedAtTime(clock.nowUtc().plusDays(1));
-    assertThat(afterTransfer.getTransferData().getTransferStatus())
-        .isEqualTo(TransferStatus.SERVER_APPROVED);
     assertThat(afterTransfer.getCurrentSponsorClientId()).isEqualTo("winner");
     assertThat(afterTransfer.getLastTransferTime()).isEqualTo(clock.nowUtc().plusDays(1));
   }