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.

watchtower/wtclient/client.go

@@ -141,10 +141,6 @@ type Config struct {
 	// new sessions will be requested immediately.
 	Policy wtpolicy.Policy
 
-	// PrivateTower is the net address of a private tower. The client will
-	// try to create all sessions with this tower.
-	PrivateTower *lnwire.NetAddress
-
 	// ChainHash identifies the chain that the client is on and for which
 	// the tower must be watching to monitor for breaches.
 	ChainHash chainhash.Hash
@@ -266,13 +262,6 @@ func New(config *Config) (*TowerClient, error) {
 		cfg.WriteTimeout = DefaultWriteTimeout
 	}
 
-	// Record the tower in our database, also loading any addresses
-	// previously associated with its public key.
-	tower, err := cfg.DB.CreateTower(cfg.PrivateTower)
-	if err != nil {
-		return nil, err
-	}
-
 	// Next, load all candidate sessions and towers from the database into
 	// the client. We will use any of these session if their policies match
 	// the current policy of the client, otherwise they will be ignored and