Eliminate proto_tree_add_text from packet-arp.c

Change-Id: Ie9596df97a135b411693eefc39fdbd03316166a0
Reviewed-on: https://code.wireshark.org/review/4828
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>