[TASK] Add a couple of code cleanups for phpstan

- Add phpdoc for variables to let phpstan know their type
- Do not use ::class syntax for PHP 7.4 classes (yet)
- Add missing class properties
- Do not unset non-existing variables

Releases: master
Resolves: #90207
Change-Id: I01ae756c03f7dfc286bedd2577979e0d79ca3d9b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63039


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>