refactor(netx): the TimeoutDialer is useless (#366)

We already configure a timeout in the underlying dialer, hence
there's no point in keeping the TimeoutDialer around.

Part of https://github.com/ooni/probe/issues/1507

internal/engine/netx/dialer/system_test.go

@@ -3,11 +3,12 @@ package dialer
 import (
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/ooni/psiphon/oopsi/golang.org/x/net/context"
 )
 
-func TestSystemDialer(t *testing.T) {
+func TestSystemDialerWorks(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel() // fail immediately
 	conn, err := Default.DialContext(ctx, "tcp", "8.8.8.8:853")
@@ -18,3 +19,10 @@ func TestSystemDialer(t *testing.T) {
 		t.Fatal("expected nil conn here")
 	}
 }
+
+func TestUnderlyingDialerHasTimeout(t *testing.T) {
+	expected := 15 * time.Second
+	if underlyingDialer.Timeout != expected {
+		t.Fatal("unexpected timeout value")
+	}
+}