[CLEANUP] Remove redundant code in Persistence

The getPlainValue method is consolidated in the DataMapper.

Resolves: #57255
Releases: 6.3
Change-Id: Ic35fd2768a1431f260c99d6faf8ad4e0a1754ea3
Reviewed-on: https://review.typo3.org/28576
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Markus Klein
Tested-by: Markus Klein