[BUGFIX] No value property in MM handling

The current handling of MM relations takes into account a
"mapping.value" property which does not exist and does not
make any sense either. Remove the whole thing.

Resolves: #59240
Releases: 2.3
Change-Id: I07ca2a70bbadf5b46f0fc644a4be1ad7f0a61747
Reviewed-on: https://review.typo3.org/30496
Reviewed-by: Francois Suter
Tested-by: Francois Suter