[TASK] Cleanup variables inside RteHtmlParser

Various cleanings to understand the code better.

Resolves: #75709
Releases: master
Change-Id: Id602fa64727508defc5859077fb821b773e1f922
Reviewed-on: https://review.typo3.org/47729


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>