Modify MM_Digitization to use MMSimHit in MM digits production. (ATLASSIM-3755)

Jing Chenrequested to merge
jichen/athena:NSWMMDigit1302 into 21.3

MMSinHit had been added into 21.3. Modify MM_Digitization package to produce digits from MMSimHit.

Edited by Jing Chen

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