To check to see if you need to delete your Additional NameNode, on the Ambari Server host:

curl -u <AMBARI_USER>:<AMBARI_PW> -H "X-Requested-By: ambari" -i -X GET <AMBARI_PROTO>://localhost:<AMBARI_PORT>/api/v1/clusters/<CLUSTER_NAME>/host_components?HostRoles/component_name=NAMENODE

If the items array contains two NameNodes, the Additional NameNode must be deleted.

To delete the Additional NameNode that was set up for HA, on the Ambari Server host:

curl -u <AMBARI_USER>:<AMBARI_PW> -H "X-Requested-By: ambari" -i -X DELETE <AMBARI_PROTO>://localhost:<AMBARI_PORT>/api/v1/clusters/<CLUSTER_NAME>/hosts/<ADDITIONAL_NAMENODE_HOSTNAME>/host_components/NAMENODE

Verify that the Additional NameNode has been deleted:

curl -u <AMBARI_USER>:<AMBARI_PW> -H "X-Requested-By: ambari" -i -X GET <AMBARI_PROTO>://localhost:<AMBARI_PORT>/api/v1/clusters/<CLUSTER_NAME>/host_components?HostRoles/component_name=NAMENODE

This should return an items array that shows only one NameNode.