peer: remove unused lastNMessages map

This map was added very early on as a possible path to implement proper
retransmission. However, we now have a proper persistent retransmission
sub-system being proposed as a PR, therefore we no longer have any use
for this.

peer.go

@@ -96,9 +96,6 @@ type peer struct {
 	inbound bool
 	id      int32
 
-	// For purposes of detecting retransmits, etc.
-	lastNMessages map[lnwire.Message]struct{}
-
 	// This mutex protects all the stats below it.
 	sync.RWMutex
 	timeConnected time.Time
@@ -184,8 +181,6 @@ func newPeer(conn net.Conn, connReq *connmgr.ConnReq, server *server,
 
 		server: server,
 
-		lastNMessages: make(map[lnwire.Message]struct{}),
-
 		sendQueueSync: make(chan struct{}, 1),
 		sendQueue:     make(chan outgoinMsg, 1),
 		outgoingQueue: make(chan outgoinMsg, outgoingQueueLen),