[TASK] Remove leftover SIGNAL_PostProcessTreeData constant

Constant should have been removed within #91474

Resolves: #91540
Related: #91474
Releases: master
Change-Id: I45081cbe71c47acaa0ce576100bb151bc130da19
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64611


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>