Remove all remaining usages of the drupal_ajax accept header

Problem/Motivation

Can we get a followup to remove the last remnants of drupal-ajax (used in tests alot) and it's drupal- friends. Or maybe it should be cleanup here? We still have things like:

$response = $this->drupalPost('editor/' . 'node/1/body/en/full', 'application/vnd.drupal-ajax', array());
and

// We set up a request so it looks like an request in the live preview.
$request = new Request();
$request->setFormat('drupal_ajax', 'application/vnd.drupal-ajax');
$request->headers->set('Accept', 'application/vnd.drupal-ajax');

Proposed resolution

Remaining tasks

User interface changes

API changes