[TASK] Cleanup runtimeActivatedPackages deprecation removal

Remove a stale comment wrt PackageManager->activePackageDuringRuntime()
as that method has been removed in #91477. Drop the setSingletonInstance
workaround which was only needed for the runtime activated packages
feature to work.

Also remove some left over runtimeActivatedPackages logic in various
places.

Resolves: #91500
Related: #91477
Related: #91473
Releases: master
Change-Id: Ia2830c230e2a8b7443bb24a79c81dbf7f2f86226
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64588


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>