Revert "server: make sendToPeer async"

This reverts commit b7704e2de3.

The root issue was fixed by commit
f4e7c36c80. As a result, this commit is no
longer needed.

server.go

@@ -908,7 +908,7 @@ func (s *server) sendToPeer(target *btcec.PublicKey, msgs []lnwire.Message) erro
 		return err
 	}
 
-	go s.sendPeerMessages(targetPeer, msgs)
+	s.sendPeerMessages(targetPeer, msgs)
 
 	return nil
 }