multi: only allow specifying towers to TowerClient through RPC

With the introduction of the WatchtowerClient RPC subserver, the lnd
configuration flag to specify private watchtowers for the client is no
longer needed and can lead to confusion upon users. Therefore, we remove
the flag completely, and only rely on the watchtower client being active
through a new --wtclient.active flag.

lnd.go

@@ -340,7 +340,7 @@ func Main() error {
 	// If the watchtower client should be active, open the client database.
 	// This is done here so that Close always executes when lndMain returns.
 	var towerClientDB *wtdb.ClientDB
-	if cfg.WtClient.IsActive() {
+	if cfg.WtClient.Active {
 		var err error
 		towerClientDB, err = wtdb.OpenClientDB(graphDir)
 		if err != nil {