cmd/derper: fix embarassing bug introduced in earlier refactor

The remove hook implementation was copy/pasted from the line above and
I didn't change the body, resulting in packet forwarding routes never
being removed.

Fortunately we weren't using this path yet, but it led to stats being
off, and (very) slow memory growth.

cmd/derper/mesh.go

@@ -39,7 +39,7 @@ func startMeshWithHost(s *derp.Server, host string) error {
 	}
 	c.MeshKey = s.MeshKey()
 	add := func(k key.Public) { s.AddPacketForwarder(k, c) }
-	remove := func(k key.Public) { s.AddPacketForwarder(k, c) }
+	remove := func(k key.Public) { s.RemovePacketForwarder(k, c) }
 	go c.RunWatchConnectionLoop(s.PublicKey(), add, remove)
 	return nil
 }