Revert "wgengine/router,util/kmod: load & log xt_mark"

This reverts commit 8d6793fd70.

Reason: breaks Android build (cgo/pthreads addition)

We can try again next cycle.

Change-Id: I5e7e1730a8bf399a8acfce546a6d22e11fb835d5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/router/router_linux.go

@@ -27,14 +27,11 @@ import (
 	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/preftype"
-	"tailscale.com/util/kmod"
 	"tailscale.com/util/multierr"
 	"tailscale.com/version/distro"
 	"tailscale.com/wgengine/monitor"
 )
 
-var disableModprobe = envknob.Bool("TS_DISABLE_MODPROBE")
-
 const (
 	netfilterOff      = preftype.NetfilterOff
 	netfilterNoDivert = preftype.NetfilterNoDivert
@@ -178,14 +175,6 @@ func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, linkMon *monit
 		}
 	}
 
-	if !disableModprobe {
-		// xt_mark is required, so attempt to load it, and log errors that occur.
-		_, err := kmod.EnsureModule("xt_mark")
-		if err != nil {
-			r.logf("ensure module xt_mark: %s", err)
-		}
-	}
-
 	return r, nil
 }