derp: remove "fine for now" intentional slow memory leak from derp server

It was once believed that it might be useful. It wasn't. We never used it.

Remove it so we don't slowly leak memory.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_server.go

@@ -130,12 +130,11 @@ type Server struct {
 	// known peer in the network, as specified by a running tailscaled's client's local api.
 	verifyClients bool
 
-	mu          sync.Mutex
-	closed      bool
-	netConns    map[Conn]chan struct{} // chan is closed when conn closes
-	clients     map[key.Public]*sclient
-	clientsEver map[key.Public]bool // never deleted from, for stats; fine for now
-	watchers    map[*sclient]bool   // mesh peer -> true
+	mu       sync.Mutex
+	closed   bool
+	netConns map[Conn]chan struct{} // chan is closed when conn closes
+	clients  map[key.Public]*sclient
+	watchers map[*sclient]bool // mesh peer -> true
 	// clientsMesh tracks all clients in the cluster, both locally
 	// and to mesh peers.  If the value is nil, that means the
 	// peer is only local (and thus in the clients Map, but not
@@ -189,7 +188,6 @@ func NewServer(privateKey key.Private, logf logger.Logf) *Server {
 		packetsDroppedReason: metrics.LabelMap{Label: "reason"},
 		packetsDroppedType:   metrics.LabelMap{Label: "type"},
 		clients:              map[key.Public]*sclient{},
-		clientsEver:          map[key.Public]bool{},
 		clientsMesh:          map[key.Public]PacketForwarder{},
 		netConns:             map[Conn]chan struct{}{},
 		memSys0:              ms.Sys,
@@ -361,7 +359,6 @@ func (s *Server) registerClient(c *sclient) {
 		go old.nc.Close()
 	}
 	s.clients[c.key] = c
-	s.clientsEver[c.key] = true
 	if _, ok := s.clientsMesh[c.key]; !ok {
 		s.clientsMesh[c.key] = nil // just for varz of total users in cluster
 	}
@@ -1342,7 +1339,6 @@ func (s *Server) expVarFunc(f func() interface{}) expvar.Func {
 // ExpVar returns an expvar variable suitable for registering with expvar.Publish.
 func (s *Server) ExpVar() expvar.Var {
 	m := new(metrics.Set)
-	m.Set("counter_unique_clients_ever", s.expVarFunc(func() interface{} { return len(s.clientsEver) }))
 	m.Set("gauge_memstats_sys0", expvar.Func(func() interface{} { return int64(s.memSys0) }))
 	m.Set("gauge_watchers", s.expVarFunc(func() interface{} { return len(s.watchers) }))
 	m.Set("gauge_current_connections", &s.curClients)