Decouple SessionMetadata and TransportCredentials

TransportCredentials are per-request, not per-session, and
there's no reason to carry them within SessionMetadata.

While I'm in here, get rid of "null" credentials.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125202213

java/google/registry/flows/StatelessRequestSessionMetadata.java

@@ -68,11 +68,6 @@ public class StatelessRequestSessionMetadata extends SessionMetadata {
     throw new UnsupportedOperationException();
   }
 
-  @Override
-  public void setTransportCredentials(TransportCredentials credentials) {
-    throw new UnsupportedOperationException();
-  }
-
   @Override
   protected void setProperty(String key, Object value) {
     throw new UnsupportedOperationException();