Remove theme_html_tag() from the theme system

Summary

html_tag is no longer available as a theme function. It has been converted to a pre-render callback instead to simplify and improve performance.

API change

Any calls to theme('html_tag'); or within a render array '#theme' => 'html_tag' need to be converted to '#type' => 'html_tag'. Drupal 8 core does not use this, but contributed modules need to note this.

Before:

$output = array(
  '#theme' => 'html_tag',
  '#tag' => 'meta',
  '#attributes' => array(
    'name' => 'Generator',
    'content' => 'Drupal 8',
  ),
);

After:

$output = array(
  '#type' => 'html_tag',
  '#tag' => 'meta',
  '#attributes' => array(
    'name' => 'Generator',
    'content' => 'Drupal 8',
  ),
);

Before:

$meta_generator = array(
  '#tag' => 'meta',
  '#attributes' => array(
    'name' => 'Generator',
    'content' => 'Drupal 8',
  ),
);
$output = theme('html_tag', array('element' => $meta_generator));

After:

$meta_generator = array(
  '#type' => 'html_tag',
  '#tag' => 'meta',
  '#attributes' => array(
    'name' => 'Generator',
    'content' => 'Drupal 8',
  ),
);
$output = drupal_render($meta_generator);