Zeroing MC event weights on old-style EventInfo

Merged Zach Marshall requested to merge zmarshal/athena:21.2_OldEventInfoZeroWeight into 21.2

This solves the duplication of information in derivations between the old-style EventInfo and the xAOD::EventInfo. The latter should be the exclusive source for the information, and is used by all the PMG tools. Therefore, we're clearing the event weights on the old-style EventInfo. In release 22, we can simply outright drop the old-style EventInfo, but in release 21 that will result in files that can't be read by athena (or the derivation framework, for example). By putting this change into the TP converter in 21.2, we get the change for all derivations immediately.

This change was discussed with @jcatmore and @elmsheus . FYI: @ewelina , @lcorpe , @amoroso , @cgutscho .

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