tstun: remove buggy-looking log line.

This log line looks buggy, even though lacking a filter is expected during bringup.
We already know if we forget to SetFilter: it breaks the magicsock test,
so no useful information is lost.

Resolves #559.

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/tstun/tun.go

@@ -109,7 +109,7 @@ type TUN struct {
 
 func WrapTUN(logf logger.Logf, tdev tun.Device) *TUN {
 	tun := &TUN{
-		logf: logf,
+		logf: logger.WithPrefix(logf, "tstun: "),
 		tdev: tdev,
 		// bufferConsumed is conceptually a condition variable:
 		// a goroutine should not block when setting it, even with no listeners.
@@ -221,7 +221,6 @@ func (t *TUN) filterOut(buf []byte) filter.Response {
 	filt, _ := t.filter.Load().(*filter.Filter)
 
 	if filt == nil {
-		t.logf("tstun: warning: you forgot to use SetFilter()! Packet dropped.")
 		return filter.Drop
 	}
 
@@ -301,7 +300,6 @@ func (t *TUN) filterIn(buf []byte) filter.Response {
 	filt, _ := t.filter.Load().(*filter.Filter)
 
 	if filt == nil {
-		t.logf("tstun: warning: you forgot to use SetFilter()! Packet dropped.")
 		return filter.Drop
 	}