pg_dump: Always dump subscriptions NOCONNECT

This removes the pg_dump option --no-subscription-connect and makes it
the default.  Dumping a subscription so that it activates right away
when restored is not very useful, because the state of the publication
server is unclear.

Discussion: https://www.postgresql.org/message-id/e4fbfad5-c6ac-fd50-6777-18c84b34eb2f@2ndquadrant.com