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/httptransport/http3transport.go

@@ -1,6 +1,7 @@
 package httptransport
 
 import (
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
@@ -9,12 +10,11 @@ import (
 // Deprecation warning
 //
 // New code should use netxlite.NewHTTP3Transport instead.
-func NewHTTP3Transport(config Config) RoundTripper {
+func NewHTTP3Transport(config Config) model.HTTPTransport {
 	// Rationale for using NoLogger here: previously this code did
 	// not use a logger as well, so it's fine to keep it as is.
 	return netxlite.NewHTTP3Transport(&NoLogger{},
-		netxlite.NewQUICDialerFromContextDialerAdapter(config.QUICDialer),
-		config.TLSConfig)
+		config.QUICDialer, config.TLSConfig)
 }
 
 type NoLogger struct{}