Drop pre-existing subscriptions from the converted subscriber.

Drop pre-existing subscriptions from the converted subscriber.

We don't need the pre-existing subscriptions on the newly formed
subscriber by using pg_createsubscriber. The apply workers corresponding
to these subscriptions can connect to other publisher nodes and either get
some unwarranted data or can lead to ERRORs in connecting to such nodes.

Author: Kuroda Hayato
Reviewed-by: Amit Kapila, Shlok Kyal, Vignesh C
Backpatch-through: 17
Discussion: http://postgr.es/m/OSBPR01MB25526A30A1FBF863ACCDDA3AF5C92@OSBPR01MB2552.jpnprd01.prod.outlook.com