Eliminate proto_tree_add_text from some dissectors.

Other minor cleanup while in the neighborhood.

Change-Id: Ib76f4a9f89b5933425760af0a980c6a549031b8f
Reviewed-on: https://code.wireshark.org/review/3537
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>