Use Writeable for ChannelMonitor instead of a specific function.

Use Writeable for ChannelMonitor instead of a specific function.

There's no reason to have ChannelMonitor::write_for_disk instead of
just using the Writeable trait anymore. Previously, it was used to
differentiate with `write_for_watchtower`, but support for
watchtower-mode ChannelMonitors was never completed and the partial
bits were removed long ago.

This has the nice benefit of hitting the custom Writeable codepaths
in C bindings instead of trying to hit trait-generics paths.