[TASK] Remove leftover unused code in various PHP classes

Due to removal of functionality and code in recent TYPO3 versions,
internal code can now be removed:

- PermissionAjaxController->getBackendUser()
- RteHtmlParser->TS_AtagToAbs (never called)
- PageRenderer->extOnReadyCode property
- SetupModuleController
  - used several properties from the now removed "simulateUser"
     code which is not part of this module anymore.
  - At the same time, various "init" and "main" methods which are now protected are
    reordered to reflect the set up request workflow better.

Resolves: #87991
Releases: master
Change-Id: I197a13ff64b3cb006730d09120d0bd8d5c5c65f0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60320


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>