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.

wgengine/watchdog_test.go

@@ -11,15 +11,10 @@ import (
 	"testing"
 	"time"
 
-	"tailscale.com/net/netns"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/tstun"
 )
 
-func init() {
-	netns.TestOnlySkipPrivilegedOps()
-}
-
 func TestWatchdog(t *testing.T) {
 	t.Parallel()