Stop completing monitor udpate completion actions when claiming on a closed channel

[TheBlueMatt]

Remove this hunk and track them globally:

        // Note that we do process the completion action here. This totally could be a
        // duplicate claim, but we have no way of knowing without interrogating the
        // `ChannelMonitor` we've provided the above update to. Instead, note that `Event`s are
        // generally always allowed to be duplicative (and it's specifically noted in
        // `PaymentForwarded`).
        self.handle_monitor_update_completion_actions(completion_action(None));

[TheBlueMatt]

Slipping this, the documentation updates in #2562 explicitly state that the async mode is not yet fully supported, which is a reference to this and one other issue involving MPP claims across channels hitting tail latency implying loss of preimage on one channel before other channels hit disk.