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/internal/sessionresolver/sessionresolver.go

@@ -110,6 +110,11 @@ func (r *Resolver) Stats() string {
 	return fmt.Sprintf("sessionresolver: %s", string(data))
 }
 
+// LookupHTTPS implements Resolver.LookupHTTPS.
+func (r *Resolver) LookupHTTPS(ctx context.Context, domain string) (*model.HTTPSSvc, error) {
+	return nil, errors.New("not implemented")
+}
+
 // ErrLookupHost indicates that LookupHost failed.
 var ErrLookupHost = errors.New("sessionresolver: LookupHost failed")