[TASK] Remove dead and outdated check for TYPO3_PATH_WEB constant

The check for the constant TYPO3_PATH_WEB was an experiment,
to allow a document root with public assets only, but
this never worked out and was never really used.

The check can safely be removed, as it won't do anything
useful anyway.

Also adapt a comment in SystemEnvironmentBuilder
which still refers to this variable/constant and its
usage in the helhum/typo3-secure-web package.
This statement can be removed as helhum/typo3-secure-web
is not actually using that constant/envvar.

Resolves: #91215
Related: #63025
Releases: master
Change-Id: I1b44ba97f4654d1e4e200d7ad1a7c228dba48c25
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64330


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>