Remove drupal_set_title() from tests and documentation

First drafted patch.

1. +++ b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
   @@ -77,8 +77,6 @@ public function preparePage(HtmlPage $page, &$page_array) {
   -      // drupal_set_title() already ensured security, so not letting the
   -      // title pass through would cause double escaping.
          $page->setTitle($title, PASS_THROUGH);
   

   Why would you remove this comment? It's still relevant & useful.

2. +++ b/core/modules/system/lib/Drupal/system/Tests/System/PageTitleTest.php
   @@ -61,14 +61,8 @@ function tearDown() {
   -    // drupal_set_title's $filter is Title::CHECK_PLAIN by default, so the title should be
   -    // returned with check_plain().
   -    drupal_set_title($title, Title::CHECK_PLAIN);
   -    $this->assertTrue(strpos(drupal_get_title(), '<em>') === FALSE, 'Tags in title converted to entities when $output is Title::CHECK_PLAIN.');
   -    // drupal_set_title's $filter is passed as PASS_THROUGH, so the title should be
   -    // returned with HTML.
   -    drupal_set_title($title, PASS_THROUGH);
   -    $this->assertTrue(strpos(drupal_get_title(), '<em>') !== FALSE, 'Tags in title are not converted to entities when $output is PASS_THROUGH.');
   +    $this->assertTrue(strpos(Title::CHECK_PLAIN, '<em>') === FALSE, 'Tags in title converted to entities when $output is Title::CHECK_PLAIN.');
   +    $this->assertTrue(strpos(PASS_THROUGH, '<em>') !== FALSE, 'Tags in title are not converted to entities when $output is PASS_THROUGH.');
   

   I'm afraid these changes are completely nonsensical.

   You're no longer testing what the results of a function call are, you're simply checking what the values of a few variables is.