Correct manual shutdown detection on channel closure

Correct manual shutdown detection on channel closure

In 5e874c3dc9cf1606a3cbbccab3a0d25089a97280 we'd intended to not
reveal the dummy funding transaction in `Event::DiscardFunding`.
However, instead of looking at the channel that was just closed,
the logic only looks at any other channels which were funded as a
part of the same batch. Because manually-funded transactions
cannot currently be done for batch funding, this was actually dead
code, preventing the new changes from taking effect.