Don't generate a `ChannelMonitorUpdate` for closed chans on shutdown

Don't generate a `ChannelMonitorUpdate` for closed chans on shutdown

The `Channel::get_shutdown` docs are very clear - if the channel
jumps to `Shutdown` as a result of not being funded when we go to
initiate shutdown we should not generate a `ChannelMonitorUpdate`
as there's no need to bother with the shutdown script - we're
force-closing anyway.

However, this wasn't actually implemented, potentially causing a
spurious monitor update for no reason.