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

Nicolas Koehlerrequested to merge
nkoehler/athena:muSimEvt into 21.3

Hi,

this MR fixes some Unused class rule warnings in MuonSimEvent since the MMSimHitCollection.h is not included in MuonSimEventDict.h. Actually, it is also not included in master, so sweeping to master and 21.9. This MR fixes ATLASSIM-4476.

Best, Nico

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