Only generate an `Event::DiscardFunding` when we need to

Only generate an `Event::DiscardFunding` when we need to

5e874c3dc9cf1606a3cbbccab3a0d25089a97280 changed
`Event::DiscardFunding` to not include a dummy transaction when we
were funded without a full funding tx, but in doing so started
generating `DiscardFunding` events on every channel closure rather
than only when there's actually still a pending funding broadcast.

This restores the previous behavior to only generate the event when
we should actually discard the funding tx.