[TASK] Remove obsolete `.t3-icon` class

As FontAwesome was removed and all icons are rendered via the Icon API
now, the CSS class `.t3-icon` became obsolete and is now removed.

Resolves: #98127
Releases: main
Change-Id: Iaf23ae71aa4a3856fe91f6265ccecc82e7190979
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75423


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>