watchtower: use ECDH interface for watchtower session

watchtower/wtmock/keyring.go

@@ -20,29 +20,6 @@ func NewSecretKeyRing() *SecretKeyRing {
 	}
 }
 
-// DerivePrivKey derives the private key for a given key descriptor. If
-// this method is called twice with the same argument, it will return the same
-// private key.
-func (m *SecretKeyRing) DerivePrivKey(
-	desc keychain.KeyDescriptor) (*btcec.PrivateKey, error) {
-
-	m.mu.Lock()
-	defer m.mu.Unlock()
-
-	if key, ok := m.keys[desc.KeyLocator]; ok {
-		return key, nil
-	}
-
-	privKey, err := btcec.NewPrivateKey(btcec.S256())
-	if err != nil {
-		return nil, err
-	}
-
-	m.keys[desc.KeyLocator] = privKey
-
-	return privKey, nil
-}
-
 // DeriveKey attempts to derive an arbitrary key specified by the
 // passed KeyLocator. This may be used in several recovery scenarios,
 // or when manually rotating something like our current default node