auto-remove redeemed channel backups

This restores commit 02e4569086.
That commit was reverted because I wrongly assumed that it
would break lnworker.has_conflicting_backup_with.

electrum/lnchannel.py

@@ -351,6 +351,9 @@ class AbstractChannel(Logger, ABC):
 
         if self.get_state() == ChannelState.CLOSED and not keep_watching:
             self.set_state(ChannelState.REDEEMED)
+            if self.lnworker and self.is_backup():
+                # auto-remove redeemed backups
+                self.lnworker.remove_channel_backup(self.channel_id)
 
     @property
     def sweep_address(self) -> str: