TRUTH3 size reduction

Removing extra copy of event weights

There are currently two identical copies of the event weights stored for
each event: one as a part of the TruthEvent, and one as a part of the
EventInfo. The official weight access tool uses the one from the
EventInfo, so we should keep that one and remove the other.

Removing MET_TruthRegions from default content

MET_TruthRegions holds detailed breakdowns of truth particles
contributing to MET by detector region (barrel, endcap, FCAL). While
this is interesting, it is far beyond what normal analyses use for
investigating MET. We don't need this in every derivation format.

Removing old-style event info from TRUTH3

This contains only duplicate information, which we don't really need in
a truth derivation, and uses significant space in the case of events
with multiple weights.

Removing legacy TRUTH3 from DerivationFrameworkSUSY

The real one should remain in DerivationFrameworkMCTruth

Total savings: 25-30% of disk space, depending on the input file.

Next target: photons.

FYI: @lcorpe , @ewelina , @jcatmore