[!!!][TASK] Remove custom userfunc of DataMapper->getPlainValue

DataMapper's hidden functionality of having a callback functionality
to modify a string before storing it in the database. This was used
for database abstraction with $TYPO3_DB ("fullQuoteStr"), which is now
obsolete for Extbase in general.

Thus, the code can be removed.

Resolves: #83241
Releases: master
Change-Id: Ic09c7572a19da570aebee969dd836371cddb5d43
Reviewed-on: https://review.typo3.org/54837


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Gorges <daniel.gorges@b13.de>
Tested-by: Daniel Gorges <daniel.gorges@b13.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>