Removing warning message from MM PRD Cnv, and fix chan Id in the converters

Closed Stefano Rosati requested to merge rosati/athena:mm_cnv into 21.3

Fixing the MM and STGC transient<->persistent converters: store the channel compact identifiers instead of the difference between the channel and the collection identifier. Remove a warning message not needed anymore.

This fix will break backwards compatibility in reading old NSW PRD, on the other hand that's not a problem because the validation of samples produced until now can go on nevertheless, and not many samples have been produced until now.

Edited by Stefano Rosati

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