lnsweep: safer maybe_reveal_preimage_for_htlc, add "is_preimage_public"

"When should we reveal preimages onchain?"
This commit tries to simplify the thinking by making the observation:
- we can reveal preimages (actually in any context) if they are already public
- a preimage is public if any other lightning node knows it besides us
  - if we learn the preimage from another LN node, it is public
  - if we send update_fulfill_htlc, it becomes public
  - if we see a preimage onchain, it is public

- in lnsweep._maybe_reveal_preimage_for_htlc:
  - partial mpp check is not relevant if preimage is already public
  - let's just always do KeepWatchingTXO, for sanity/safety

Co-authored-by: ThomasV <thomasv@electrum.org>

electrum/lnpeer.py

@@ -1983,7 +1983,6 @@ class Peer(Logger, EventListener):
                 f"chan={chan.get_id_for_log()}. {htlc_id=}. {chan.get_state()=!r}. {chan.peer_state=!r}")
             return
         chan.receive_htlc_settle(preimage, htlc_id)  # TODO handle exc and maybe fail channel (e.g. bad htlc_id)
-        self.lnworker.save_preimage(payment_hash, preimage)
         self.maybe_send_commitment(chan)
 
     def on_update_fail_malformed_htlc(self, chan: Channel, payload):