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/magicsock/magicsock_test.go

@@ -26,7 +26,6 @@ import (
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/derp/derpmap"
-	"tailscale.com/net/netns"
 	"tailscale.com/net/stun/stuntest"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
@@ -36,10 +35,6 @@ import (
 	"tailscale.com/wgengine/tstun"
 )
 
-func init() {
-	netns.TestOnlySkipPrivilegedOps()
-}
-
 // WaitReady waits until the magicsock is entirely initialized and connected
 // to its home DERP server. This is normally not necessary, since magicsock
 // is intended to be entirely asynchronous, but it helps eliminate race