pg_createsubscriber: Remove replication slot check on primary

pg_createsubscriber: Remove replication slot check on primary

It used to check if the replication slot exists and is active on
primary.  This check might fail on slow hosts because the replication
slot might not be active at the time of this check.

The current code obtains the replication slot name from the
primary_slot_name on standby and assumes the replication slot exists
and is active on primary.  If it doesn't exist, this tool will log an
error and continue.

Author: Euler Taveira <euler.taveira@enterprisedb.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://www.postgresql.org/message-id/776c5cac-5ef5-4001-b1bc-5b698bc0c62a%40app.fastmail.com