all: cleanup unused code, part 1 (#10661)

Run `staticcheck` with `U1000` to find unused code. This cleans up about
a half of it. I'll do the other half separately to keep PRs manageable.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

net/tstun/wrap_test.go

@@ -258,7 +258,6 @@ func TestWriteAndInject(t *testing.T) {
 	chtun, tun := newChannelTUN(t.Logf, false)
 	defer tun.Close()
 
-	const size = 2 // all payloads have this size
 	written := []string{"w0", "w1"}
 	injected := []string{"i0", "i1"}