multi: remove pending channel from Brontide when funding flow failed

This commit adds a new interface method, `RemovePendingChannel`, to be
used when the funding flow is failed after calling `AddPendingChannel`
such that the Brontide has the most up-to-date view of the active
channels.

funding/manager_test.go

@@ -327,6 +327,10 @@ func (n *testNode) AddPendingChannel(_ lnwire.ChannelID,
 	return nil
 }
 
+func (n *testNode) RemovePendingChannel(_ lnwire.ChannelID) error {
+	return nil
+}
+
 func createTestWallet(cdb *channeldb.ChannelStateDB, netParams *chaincfg.Params,
 	notifier chainntnfs.ChainNotifier, wc lnwallet.WalletController,
 	signer input.Signer, keyRing keychain.SecretKeyRing,