[!!!][TASK] Remove legacy SoftReferenceIndex

The old implementation and all backwards
compatibility layers for SoftReferenceIndex have
been dropped.

Resolves: #96145
Related: #94687
Releases: main
Change-Id: Ifa3bacbf3a7dc1ffad5acca852f1a2c25ac87b40
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72381


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>