Remove EntityBrowser::getOption

The method EntityBrowser::getOption() is no longer needed. It was added to fix a regression where ajaxEnabled() which should have been used to determine if a view display should use ajax stopped working, now that the core issue is fixed, we can remove the method.

In this method in Drupal\entity_browser\Plugin\views\display\EntityBrowser:

// @todo remove upon resolution of https://www.drupal.org/node/2904798 →
// This overrides getOption() instead of ajaxEnabled() because
// \Drupal\views\Controller\ViewAjaxController::ajaxView() currently calls
// that directly.