Eliminate proto_tree_add_text

Also some other cleanup to simplify dissector logic.

Change-Id: Ia84300b7eadafe292361014dd0e565ec250cae44
Reviewed-on: https://code.wireshark.org/review/5311
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>