[TASK] Remove sysext/install index.php comment

The comment in ext:install/Resources/Private/Php/install.php
has been established with the very first rewrite of the
install tool in 2013.
After a series of further rewrites and adaptions it is
nowadays pretty much completely outdated and fits no
good purpose anymore.
Let's get rid of it.

Resolves: #99850
Releases: main
Change-Id: Id778b6f4cfaa23c725d2b61ef8e9b43c86a2db23
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77716


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Benni Mack <benni@typo3.org>