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.

discovery/mock_test.go

@@ -69,6 +69,10 @@ func (p *mockPeer) AddPendingChannel(_ lnwire.ChannelID,
 	return nil
 }
 
+func (p *mockPeer) RemovePendingChannel(_ lnwire.ChannelID) error {
+	return nil
+}
+
 // mockMessageStore is an in-memory implementation of the MessageStore interface
 // used for the gossiper's unit tests.
 type mockMessageStore struct {