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/session.go

@@ -51,7 +51,7 @@ type Session struct {
 	availableProbeServices   []model.OOAPIService
 	availableTestHelpers     map[string][]model.OOAPIService
 	byteCounter              *bytecounter.Counter
-	httpDefaultTransport     netx.HTTPRoundTripper
+	httpDefaultTransport     model.HTTPTransport
 	kvStore                  model.KeyValueStore
 	location                 *geolocate.Results
 	logger                   model.Logger