Eliminate proto_tree_add_text from GSM_SMS dissector.

Change-Id: Ibe189239735da2ebb1b0ce61a5af249975b38be3
Reviewed-on: https://code.wireshark.org/review/4402
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>