[TASK] Streamline linkvalidator code

This is a pre-patch to fluidize the Backend module and the
linkvalidator email as next steps.

- Remove unused functionality
- Move database calls to linkvalidator table into BrokenLinkRepository (incl. json_decode)
- Fix PHP comments and method argument handling

Resolves: #90314
Releases: master
Change-Id: I1850b182577c72a3ad97247fb33bc164ea5e7925
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63170


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>