[FOLLOWUP][TASK] Remove $db property in UserAuthentication classes

This patch adds a comment why the empty constructor function in
the AbstractUserAuthentication has to stay.

Resolves: #76353
Releases: master
Change-Id: I9634d15579fed97a3f2f553a317117bad57eec5c
Reviewed-on: https://review.typo3.org/48437


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>