[RAPPS] Fallback to legacy "size" entry is not longer needed

on 2021-10-03 Mark Jansen deleted all old size entries from the rapps database via
https://git.reactos.org/?p=rapps-db.git;a=commit;h=551e8cb95716f46e05c3be08996467bd93b1936f

So it makes no sense to keep that fallback-code in RAPPS available.cpp RetrieveSize() func.