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/cmd/jafar/uncensored/uncensored.go

@@ -17,7 +17,7 @@ import (
 
 // Client is DNS, HTTP, and TCP client.
 type Client struct {
-	dnsClient     *netx.DNSClient
+	dnsClient     model.Resolver
 	httpTransport model.HTTPTransport
 	dialer        model.Dialer
 }
@@ -34,7 +34,7 @@ func NewClient(resolverURL string) (*Client, error) {
 		return nil, err
 	}
 	return &Client{
-		dnsClient:     &configuration.DNSClient,
+		dnsClient:     configuration.DNSClient,
 		httpTransport: netx.NewHTTPTransport(configuration.HTTPConfig),
 		dialer:        netx.NewDialer(configuration.HTTPConfig),
 	}, nil