lnwatcher: don't add `REDEEMED` channels

Previously lnworker called LNWatcher.add_channel() on these, and then LNWatcher itself decided to unwatch them.

-----

note: motivation related to https://github.com/spesmilo/electrum/pull/7932#discussion_r945755584 ,
where I have legacy (pre-staticremotekey) redeemed channels in a wallet.
Tracebacks:
```
E/W | lnwatcher.LNWalletWatcher.[default_wallet-LNW] | Exception in do_breach_remedy: AssertionError()
Traceback (most recent call last):
  File "...\electrum\electrum\util.py", line 1235, in wrapper
    return await func(*args, **kwargs)
  File "...\electrum\electrum\lnwatcher.py", line 443, in do_breach_remedy
    sweep_info_dict = chan.sweep_ctx(closing_tx)
  File "...\electrum\electrum\lnchannel.py", line 264, in sweep_ctx
    our_sweep_info = self.create_sweeptxs_for_our_ctx(ctx)
  File "...\electrum\electrum\lnchannel.py", line 253, in create_sweeptxs_for_our_ctx
    return create_sweeptxs_for_our_ctx(chan=self, ctx=ctx, sweep_address=self.sweep_address)
  File "...\electrum\electrum\lnchannel.py", line 757, in sweep_address
    assert self.is_static_remotekey_enabled()
AssertionError
```
```
E/W | lnwatcher.LNWalletWatcher.[default_wallet-LNW] | Exception in trigger_callbacks: AssertionError()
Traceback (most recent call last):
  File "...\electrum\electrum\util.py", line 1235, in wrapper
    return await func(*args, **kwargs)
  File "...\electrum\electrum\lnwatcher.py", line 208, in trigger_callbacks
    await callback()
  File "...\electrum\electrum\lnwatcher.py", line 225, in check_onchain_situation
    keep_watching = await self.do_breach_remedy(funding_outpoint, closing_tx, spenders)
  File "...\electrum\electrum\util.py", line 1235, in wrapper
    return await func(*args, **kwargs)
  File "...\electrum\electrum\lnwatcher.py", line 443, in do_breach_remedy
    sweep_info_dict = chan.sweep_ctx(closing_tx)
  File "...\electrum\electrum\lnchannel.py", line 264, in sweep_ctx
    our_sweep_info = self.create_sweeptxs_for_our_ctx(ctx)
  File "...\electrum\electrum\lnchannel.py", line 253, in create_sweeptxs_for_our_ctx
    return create_sweeptxs_for_our_ctx(chan=self, ctx=ctx, sweep_address=self.sweep_address)
  File "...\electrum\electrum\lnchannel.py", line 757, in sweep_address
    assert self.is_static_remotekey_enabled()
AssertionError
```

electrum/lnworker.py

@@ -758,9 +758,11 @@ class LNWallet(LNWorker):
         self.lnrater = LNRater(self, network)
 
         for chan in self.channels.values():
-            self.lnwatcher.add_channel(chan.funding_outpoint.to_str(), chan.get_funding_address())
+            if not chan.is_redeemed():
+                self.lnwatcher.add_channel(chan.funding_outpoint.to_str(), chan.get_funding_address())
         for cb in self.channel_backups.values():
-            self.lnwatcher.add_channel(cb.funding_outpoint.to_str(), cb.get_funding_address())
+            if not cb.is_redeemed():
+                self.lnwatcher.add_channel(cb.funding_outpoint.to_str(), cb.get_funding_address())
 
         for coro in [
                 self.maybe_listen(),