FastCaloSim in G4: Drop diff-pool checks for MT jobs

diff-pool checks can show differences due to the different ordering of events in the output file. This is nothing to be concerned out, so disabling them for test_AtlasG4_FastCaloSim_MT.sh and test_AtlasG4_FastCaloSim_MTvsST.sh Essentially a backport of !69612 (merged) to 24.0

Should fix ATLASSIM-7204