Drop duplicative current-local-tx storage in channel.

We now have current-local-tx broadcast ability in channel monitors
directly (for ChannelManager deserialization), so we can just use
that instead of always having the Channel store signed ready-to-go
copies of the latest local commitment transaction.

This is further kinda nice since ChannelMonitor is live and can, eg
broadcast HTLC-Success transactions immediately as they will be
generated at broadcast time instead of in advance.

Finally, this lets us clean up a tiny bit in Channel.