Qt: remove the 'local watchtower' option from GUI.

It is not realistic to expect Electrum to be used as a watchtower
in GUI mode, and possibly counter-productive (may set wrong
expectations).

A proper watchtower should be configured as a daemon. The
documentation will be updated to reflect this change.

electrum/network.py

@@ -351,7 +351,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
 
         # lightning network
         self.channel_blacklist = ChannelBlackList()
-        if self.config.get('run_local_watchtower', False):
+        if self.config.get('run_watchtower', False):
             from . import lnwatcher
             self.local_watchtower = lnwatcher.WatchTower(self)
             self.local_watchtower.start_network(self)