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_enabled.go

@@ -7,13 +7,15 @@ import (
 	"context"
 	"net"
 	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // shapingDialer ensures we don't use too much bandwidth
 // when using integration tests at GitHub. To select
 // the implementation with shaping use `-tags shaping`.
 type shapingDialer struct {
-	Dialer
+	model.Dialer
 }
 
 // DialContext implements Dialer.DialContext