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.

lnpeer/peer.go

@@ -31,6 +31,9 @@ type Peer interface {
 	// channel should fail to be added if the cancel chan is closed.
 	AddPendingChannel(cid lnwire.ChannelID, cancel <-chan struct{}) error
 
+	// RemovePendingChannel removes a pending open channel ID to the peer.
+	RemovePendingChannel(cid lnwire.ChannelID) error
+
 	// WipeChannel removes the channel uniquely identified by its channel
 	// point from all indexes associated with the peer.
 	WipeChannel(*wire.OutPoint)