[BUGFIX] FormEngine: Remove hardcoded fieldnames for subtypes_addlist

Take into account the configured fieldnames for recordType and subType
selection instead of using the hardcoded values list and list_type.

Resolves: #71212
Related: #62508
Releases: master
Change-Id: I81cad273859489464b2d902092ff81c0bd17ae10
Reviewed-on: https://review.typo3.org/44452


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>