derp,ipn/ipnlocal: stop calling rand.Seed

It's deprecated and using it gets us the old slow behavior
according to https://go.dev/blog/randv2.

> Having eliminated repeatability of the global output stream, Go 1.20
> was also able to make the global generator scale better in programs
> that don’t call rand.Seed, replacing the Go 1 generator with a very
> cheap per-thread wyrand generator already used inside the Go
> runtime. This removed the global mutex and made the top-level
> functions scale much better. Programs that do call rand.Seed fall
> back to the mutex-protected Go 1 generator.

Updates #7123

Change-Id: Ia5452e66bd16b5457d4b1c290a59294545e13291
Signed-off-by: Maisem Ali <maisem@tailscale.com>

derp/derp_server.go

@@ -69,10 +69,6 @@ func init() {
 	}
 }
 
-func init() {
-	rand.Seed(time.Now().UnixNano())
-}
-
 const (
 	perClientSendQueueDepth = 32 // packets buffered for sending
 	writeTimeout            = 2 * time.Second