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/experiment/ndt7/dial.go

@@ -8,7 +8,6 @@ import (
 
 	"github.com/gorilla/websocket"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/resolver"
 	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
@@ -34,9 +33,9 @@ func newDialManager(ndt7URL string, logger model.Logger, userAgent string) dialM
 }
 
 func (mgr dialManager) dialWithTestName(ctx context.Context, testName string) (*websocket.Conn, error) {
-	var reso resolver.Resolver = &netxlite.ResolverSystem{}
+	var reso model.Resolver = &netxlite.ResolverSystem{}
 	reso = &netxlite.ResolverLogger{
-		Resolver: netxlite.NewResolverLegacyAdapter(reso),
+		Resolver: reso,
 		Logger:   mgr.logger,
 	}
 	dlr := dialer.New(&dialer.Config{