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/resolver/system.go

@@ -1,10 +1,29 @@
 package resolver
 
-import "github.com/ooni/probe-cli/v3/internal/engine/netx/selfcensor"
+import (
+	"context"
+	"net"
+)
 
-// SystemResolver is the system resolver. It is implemented using
-// selfcensor.SystemResolver so that we can perform integration testing
-// by forcing the code to return specific responses.
-type SystemResolver = selfcensor.SystemResolver
+// SystemResolver is the system resolver.
+type SystemResolver struct{}
+
+// LookupHost implements Resolver.LookupHost.
+func (r SystemResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	return net.DefaultResolver.LookupHost(ctx, hostname)
+}
+
+// Network implements Resolver.Network.
+func (r SystemResolver) Network() string {
+	return "system"
+}
+
+// Address implements Resolver.Address.
+func (r SystemResolver) Address() string {
+	return ""
+}
+
+// Default is the resolver we use by default.
+var Default = SystemResolver{}
 
 var _ Resolver = SystemResolver{}