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.go

@@ -5,12 +5,13 @@ import (
 	"net"
 
 	"github.com/ooni/probe-cli/v3/internal/bytecounter"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // byteCounterDialer is a byte-counting-aware dialer. To perform byte counting, you
 // should make sure that you insert this dialer in the dialing chain.
 type byteCounterDialer struct {
-	Dialer
+	model.Dialer
 }
 
 // DialContext implements Dialer.DialContext