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/cmd/oohelperd/internal/webconnectivity/webconnectivity_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
-	"net"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -52,7 +51,7 @@ const requestWithoutDomainName = `{
 func TestWorkingAsIntended(t *testing.T) {
 	handler := Handler{
 		Client:            http.DefaultClient,
-		Dialer:            new(net.Dialer),
+		Dialer:            netxlite.DefaultDialer,
 		MaxAcceptableBody: 1 << 24,
 		Resolver:          &netxlite.ResolverSystem{},
 	}