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.

net/netcheck/netcheck_test.go

@@ -16,16 +16,11 @@ import (
 	"time"
 
 	"tailscale.com/net/interfaces"
-	"tailscale.com/net/netns"
 	"tailscale.com/net/stun"
 	"tailscale.com/net/stun/stuntest"
 	"tailscale.com/tailcfg"
 )
 
-func init() {
-	netns.TestOnlySkipPrivilegedOps()
-}
-
 func TestHairpinSTUN(t *testing.T) {
 	tx := stun.NewTxID()
 	c := &Client{