save_backup: do not remove deterministic LN key (see #7513)

electrum/tests/regtest/regtest.sh

@@ -150,7 +150,9 @@ if [[ $1 == "backup" ]]; then
     $alice daemon -d
     $alice load_wallet
     $alice import_channel_backup $backup
+    echo "request force close $channel1"
     $alice request_force_close $channel1
+    echo "request force close $channel2"
     $alice request_force_close $channel2
     wait_for_balance alice 0.998
 fi

electrum/wallet.py

@@ -322,7 +322,6 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
             for chan_id, chan in self.lnworker.channels.items():
                 channel_backups[chan_id.hex()] = self.lnworker.create_channel_backup(chan_id)
             new_db.put('channels', None)
-            new_db.put('lightning_xprv', None)
             new_db.put('lightning_privkey2', None)
 
         new_path = os.path.join(backup_dir, self.basename() + '.backup')