pgsql: 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: https://postgr.es/m/OSBPR01MB25526A30A1FBF863ACCDDA3AF5C92@OSBPR01MB2552.jpnprd01.prod.outlook.com

Branch
------
REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/622cb84d69be91931568bee180cae7c484a7f026

Modified Files
--------------
src/bin/pg_basebackup/pg_createsubscriber.c        | 111 ++++++++++++++++++++-
src/bin/pg_basebackup/t/040_pg_createsubscriber.pl |  14 +++
2 files changed, 120 insertions(+), 5 deletions(-)