refactor(netx): remove the self censorship mechanism (#364)

We're currently use jafar for QA and jafar is a better mechanism,
even though it is not portable outside of Linux.

This self censorship mechanism was less cool and added a bunch
of (also cognitive) complexity to netx.

If we ever want to go down a self censorship like road, we probably
want to do as little work as possible in the problem and as much
work as possible inside a helper like jafar.

Part of https://github.com/ooni/probe/issues/1591.

internal/engine/netx/dialer/system.go

@@ -0,0 +1,24 @@
+package dialer
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+// defaultNetDialer is the net.Dialer we use by default.
+var defaultNetDialer = &net.Dialer{
+	Timeout:   15 * time.Second,
+	KeepAlive: 15 * time.Second,
+}
+
+// SystemDialer is the system dialer.
+type SystemDialer struct{}
+
+// DialContext implements Dialer.DialContext
+func (d SystemDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return defaultNetDialer.DialContext(ctx, network, address)
+}
+
+// Default is the dialer we use by default.
+var Default = SystemDialer{}