cleanup(netx): remove the DNSClient type (#660)

The DNSClient type existed because the Resolver type did not
include CloseIdleConnections in its signature.

Now that Resolver includes CloseIdleConnections, the DNSClient
type has become unnecessary and can be safely removed.

See https://github.com/ooni/probe/issues/1956.

internal/engine/experiment/urlgetter/configurer.go

@@ -26,7 +26,7 @@ type Configurer struct {
 // The Configuration is the configuration for running a measurement.
 type Configuration struct {
 	HTTPConfig netx.Config
-	DNSClient  netx.DNSClient
+	DNSClient  model.Resolver
 }
 
 // CloseIdleConnections will close idle connections, if needed.
@@ -82,7 +82,7 @@ func (c Configurer) NewConfiguration() (Configuration, error) {
 		return configuration, err
 	}
 	configuration.DNSClient = dnsclient
-	configuration.HTTPConfig.BaseResolver = dnsclient.Resolver
+	configuration.HTTPConfig.BaseResolver = dnsclient
 	// configure TLS
 	configuration.HTTPConfig.TLSConfig = &tls.Config{
 		NextProtos: []string{"h2", "http/1.1"},