Remove useless createdRepoId from logging

It is no longer needed since we don't do log-replay and it's
annoying to support in the flat flows.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135805088

java/google/registry/flows/Flow.java

@@ -59,14 +59,6 @@ public abstract class Flow {
   /** Execute the business logic for this flow. */
   protected abstract EppOutput run() throws EppException;
 
-  /**
-   * Subclasses that create a resource should override this to return the repoId of the new
-   * resource.
-   */
-  protected String getCreatedRepoId() {
-    return null;
-  }
-
   protected String getClientId() {
     return sessionMetadata.getClientId();
   }