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.

htlcswitch/mock.go

@@ -678,6 +678,10 @@ func (s *mockServer) AddPendingChannel(_ lnwire.ChannelID,
 	return nil
 }
 
+func (s *mockServer) RemovePendingChannel(_ lnwire.ChannelID) error {
+	return nil
+}
+
 func (s *mockServer) WipeChannel(*wire.OutPoint) {}
 
 func (s *mockServer) LocalFeatures() *lnwire.FeatureVector {