Do not try to set DoSlimming property for SimpleMergeMcEventCollTool (ATLASSIM-6622)
Fixes legacy configuration for the case where some pile-up truth is saved. Missed update from !64049 (merged). Closes ATLASSIM-6622
Tagging @martindl
Fixes legacy configuration for the case where some pile-up truth is saved. Missed update from !64049 (merged). Closes ATLASSIM-6622
Tagging @martindl