ipn/ipnlocal: be consistent in not logging when no notify registered

Some paths already didn't. And in the future I hope to shut all the
notify funcs down end-to-end when nothing is connected (as in the
common case in tailscaled).  Then we can save some JSON encoding work.

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

ipn/ipnlocal/local.go

@@ -932,7 +932,6 @@ func (b *LocalBackend) send(n ipn.Notify) {
 	b.mu.Unlock()
 
 	if notifyFunc == nil {
-		b.logf("nil notify callback; dropping %+v", n)
 		return
 	}