MiniAOD: set track_ as transient in PackedPFCandidate

Fixing the issue reported by David on uncompressed size increase for PackedPFCandidate.
The mutable track_ should not be stored (it is anyhow compressed to negligible size being never filled, but it should not be stored)

on 100 events is now:
patPackedCandidates_packedPFCandidates__PAT. 62292.6 14063.3
to be compared with 70X numbers from david:
patPackedCandidates_packedPFCandidates__PAT. 64448.1 15698.8

@ferencek @davidlange6 @gpetruc