Mark Trap-PDU as obsolete for SNMP versions > 1

Change-Id: I967a1547eb97619042b620bb3d803a29cce9ebe1
Reviewed-on: https://code.wireshark.org/review/3920
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>