802.11: Remove proto_tree_add_text

Change-Id: I40d351e3317ff7e207e5c68a39aaedb73faf54f9
Reviewed-on: https://code.wireshark.org/review/5281
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>