Remove the code used to workaround the lack of support for stream wrappers in GDToolkit::save()

GDToolkit::save() contains the following code.

  // Work around lack of stream wrapper support in imagejpeg() and imagepng().
  if ($scheme && $this->streamWrapperManager->isValidScheme($scheme)) {
    // If destination is not local, save image to temporary local file.
    $local_wrappers = $this->streamWrapperManager->getWrappers(StreamWrapperInterface::LOCAL);
    if (!isset($local_wrappers[$scheme])) {
      $permanent_destination = $destination;
      $destination = $this->fileSystem->tempnam('temporary://', 'gd_');
    }

    // Convert stream wrapper URI to normal path.
    $destination = $this->fileSystem->realpath($destination);
  }

As the comment says, that code workarounds the lack of support for stream wrappers in imagejpeg() and imagepng(), but those functions accept a stream resource since PHP 5.4, as the Changelog section of those functions says.

5.4.0 Added support for passing a stream resource to to.

That code should be removed, since Drupal 8 doesn't support PHP 5.4 anymore, and be replaced from code that passes a stream resource to the GD functions used to save an image.