Eliminate proto_tree_add_text from packet-gtpv2.c

Change-Id: I127447941e4c926d9194373a405c82145b1392bb
Reviewed-on: https://code.wireshark.org/review/5025
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>