refactor(oohelperd): use netxlite rather than netx (#806)

The oohelperd implementation did not actually need using netx because
it was just constructing default types with logging, which is what
netxlite already does. Hence, let's avoid using netx here.

See https://github.com/ooni/probe/issues/2121

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

@@ -14,7 +14,7 @@ import (
 
 // Handler implements the Web Connectivity test helper HTTP API.
 type Handler struct {
-	Client            *http.Client
+	Client            model.HTTPClient
 	Dialer            model.Dialer
 	MaxAcceptableBody int64
 	Resolver          model.Resolver