AsgMessaging: remove duplicate code in MessageCheck

Frank Winklmeierrequested to merge
fwinkl/athena:asgmsg into master

Not sure how this happened but MessageCheck.cxx contains largely the same code as MsgHelpers.cxx and the former is unused. Must have gone wrong in one of the manual sweeps. Replace it with the version from 21.2.

cc @krumnack

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