[TASK] update the comments regarding ajax.php

There are a few leftover parts when ajax.php was used,
before the refactoring of typo3/index.php and PSR-7 AJAX Routes.

Resolves: #70251
Releases: master
Change-Id: I400fa04e08a574a5922d62a68d588220c51be0e1
Reviewed-on: http://review.typo3.org/43841


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>