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/websteps/tcp.go

@@ -4,13 +4,14 @@ import (
 	"context"
 	"net"
 
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 type TCPConfig struct {
-	Dialer   netxlite.DialerLegacy
+	Dialer   model.Dialer
 	Endpoint string
-	Resolver netxlite.ResolverLegacy
+	Resolver model.Resolver
 }
 
 // TCPDo performs the TCP check.