Fix DROP SUBSCRIPTION hang

When ALTER SUBSCRIPTION DISABLE is run in the same transaction before
DROP SUBSCRIPTION, the latter will hang because workers will still be
running, not having seen the DISABLE committed, and DROP SUBSCRIPTION
will wait until the workers have vacated the replication origin slots.

Previously, DROP SUBSCRIPTION killed the logical replication workers
immediately only if it was going to drop the replication slot, otherwise
it scheduled the worker killing for the end of the transaction, as a
result of 7e174fa7.  This, however,
causes the present problem.  To fix, kill the workers immediately in all
cases.  This covers all cases: A subscription that doesn't have a
replication slot must be disabled.  It was either disabled in the same
transaction, or it was already disabled before the current transaction,
but then there shouldn't be any workers left and this won't make a
difference.

Reported-by: Arseny Sher <a.sher@postgrespro.ru>
Discussion: https://www.postgresql.org/message-id/flat/87mv6av84w.fsf%40ars-thinkpad