Use full LArGMCfg CA in L1CaloFEXSimCfg for MC

Apparently fixes ATR-24937, i.e. a race condition between two ways of ensuring correct dependencies for data read from the input file - SGInputLoader and AthReadAlg. The latter isn't really needed in L1CaloFEXSimCfg, certainly not when this config is used standalone. However, in full HLT jobs the AthReadAlg is included by other configuration fragments which clashes with the SGInputLoader solution used previously in L1CaloFEXSimCfg. The solution is to just use that same fragment (LArGMCfg) in L1CaloFEXSimCfg.