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/netx/fake_test.go

@@ -19,6 +19,8 @@ func (d FakeDialer) DialContext(ctx context.Context, network, address string) (n
 	return d.Conn, d.Err
 }
 
+func (d FakeDialer) CloseIdleConnections() {}
+
 type FakeTransport struct {
 	Err  error
 	Func func(*http.Request) (*http.Response, error)