Eliminate proto_tree_add_text from packet-telnet.c

Change-Id: I0e8610f381e650f2c5b3f78ea927b727ec9ac62a
Reviewed-on: https://code.wireshark.org/review/5637
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>