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/netx/dialer/shaping_test.go

@@ -1,13 +1,14 @@
 package dialer
 
 import (
-	"net"
 	"net/http"
 	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestShapingDialerGood(t *testing.T) {
-	d := &shapingDialer{Dialer: &net.Dialer{}}
+	d := &shapingDialer{Dialer: netxlite.DefaultDialer}
 	txp := &http.Transport{DialContext: d.DialContext}
 	client := &http.Client{Transport: txp}
 	resp, err := client.Get("https://www.google.com/")