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/cmd/oohelperd/internal/webconnectivity/webconnectivity.go

@@ -6,7 +6,7 @@ import (
 	"io"
 	"net/http"
 
-	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/runtimex"
 	"github.com/ooni/probe-cli/v3/internal/version"
@@ -15,9 +15,9 @@ import (
 // Handler implements the Web Connectivity test helper HTTP API.
 type Handler struct {
 	Client            *http.Client
-	Dialer            netx.Dialer
+	Dialer            model.Dialer
 	MaxAcceptableBody int64
-	Resolver          netx.Resolver
+	Resolver          model.Resolver
 }
 
 func (h Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {