Eliminate proto_tree_add_text from packet-mate.c

Change-Id: I3f4b3eb23b91b219df373b3012fbefa63abfa4d3
Reviewed-on: https://code.wireshark.org/review/5350
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>