net/netns, misc tests: remove TestOnlySkipPrivilegedOps, argv checks

The netns UID check is sufficient for now. We can do something else
later if/when needed.

derp/derphttp/derphttp_test.go

@@ -15,14 +15,9 @@ import (
 	"time"
 
 	"tailscale.com/derp"
-	"tailscale.com/net/netns"
 	"tailscale.com/types/key"
 )
 
-func init() {
-	netns.TestOnlySkipPrivilegedOps()
-}
-
 func TestSendRecv(t *testing.T) {
 	const numClients = 3
 	var serverPrivateKey key.Private