Get rid of '#theme' => '' in \Drupal\views\Form\ViewsExposedForm::buildForm

Problem/Motivation

D8 codebase inherits hack that makes no sense

  public function buildForm(array $form, FormStateInterface $form_state) {
    // Don't show the form when batch operations are in progress.
    if ($batch = batch_get() && isset($batch['current_set'])) {
      return array(
        // Set the theme callback to be nothing to avoid errors in template_preprocess_views_exposed_form().'#theme' => '',
      );
    }

But template_preprocess_views_exposed_form has nothing about theme name for form
This disallows to use custom bulk actions that needs to implement own batching without confirm form

Proposed resolution

Get rid of hack

Remaining tasks

Find proper way to implement batching
Patch, test, commit

User interface changes

API changes

Data model changes