Remove the NODROP SLOT option from DROP SUBSCRIPTION

Remove the NODROP SLOT option from DROP SUBSCRIPTION

It turned out this approach had problems, because a DROP command should
not have any options other than CASCADE and RESTRICT.  Instead, always
attempt to drop the slot if there is one configured, but also add an
ALTER SUBSCRIPTION action to set the slot to NONE.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/29431.1493730652@sss.pgh.pa.us