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/bytecounter_test.go

@@ -16,7 +16,7 @@ import (
 func dorequest(ctx context.Context, url string) error {
 	txp := http.DefaultTransport.(*http.Transport).Clone()
 	defer txp.CloseIdleConnections()
-	dialer := &byteCounterDialer{Dialer: new(net.Dialer)}
+	dialer := &byteCounterDialer{Dialer: netxlite.DefaultDialer}
 	txp.DialContext = dialer.DialContext
 	client := &http.Client{Transport: txp}
 	req, err := http.NewRequestWithContext(ctx, "GET", "http://www.google.com", nil)