Eliminate proto_tree_add_text from packet-igrp.c

Change-Id: If73e3bb924036b7746d15058e9fca6f551f9538c
Reviewed-on: https://code.wireshark.org/review/4827
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>