Sweeping !31654 from 21.3 to 21.9. Fix Unused class rule warning in MuonSimEvent (ATLASSIM-4476)

Atlas Nightlybuildrequested to merge
cherry-pick-bc27550a3f-21.9 into 21.9

Fix Unused class rule warning in MuonSimEvent (ATLASSIM-4476)

See merge request !31654 (merged)

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