[TASK] Cleanup t3editor controllers and docs

EXT:t3editor has some AJAX controllers which can be simplified:
- Unused properties are removed
- Unrelated "LANG" calls can be streamlined, resulting in a lowered
  memory footprint
- Rootline resolving is done directly via the existing class
- Remove obsolete usage of PhpFrontend in phpDoc in T3editor base class

Resolves: #85081
Releases: master
Change-Id: Ia5669d161c59881e509d9b7e881cc555ef2b44c1
Reviewed-on: https://review.typo3.org/57051


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>