cleanup: remove unnecessary legacy interfaces (#656)

This diff addresses another point of https://github.com/ooni/probe/issues/1956:

> - [ ] observe that we're still using a bunch of private interfaces for common interfaces such as the `Dialer`, so we can get rid of these private interfaces and always use the ones in `model`, which allows us to remove a bunch of legacy wrappers

Additional cleanups may still be possible. The more I cleanup, the more I see
there's extra legacy code we can dispose of (which seems good?).

internal/engine/experiment.go

@@ -286,8 +286,8 @@ func (e *Experiment) OpenReportContext(ctx context.Context) error {
 	// use custom client to have proper byte accounting
 	httpClient := &http.Client{
 		Transport: &httptransport.ByteCountingTransport{
-			RoundTripper: e.session.httpDefaultTransport, // proxy is OK
-			Counter:      e.byteCounter,
+			HTTPTransport: e.session.httpDefaultTransport, // proxy is OK
+			Counter:       e.byteCounter,
 		},
 	}
 	client, err := e.session.NewProbeServicesClient(ctx)