Do not serialize `Channel::last_sent_closing_fee` to disk

We're supposed to write `Channel` to disk as if
`remove_uncommitted_htlcs_and_mark_paused` had just run, however we
were writing `last_sent_closing_fee` to disk (if it is not-None),
whereas `remove_uncommitted_htlcs_and_mark_paused` clears it.
Indeed, the BOLTs say fee "... negotiation restarts on
reconnection."