Eliminate proto_tree_add_text from packet-q933.c

Change-Id: If5b85609d3daaf66cbcc15e3127af4e7f5736e52
Reviewed-on: https://code.wireshark.org/review/4995
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>