Eliminate proto_tree_add_text from some dissectors.

Some other related cleanup.

Change-Id: I45f54032aa8318858f4ee784945b6f2ed163b6ea
Reviewed-on: https://code.wireshark.org/review/4328
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>