Remove (almost all) proto_tree_add_text calls in ISUP dissector.

Change-Id: I57a01eacaa02e45c23bb4827ae982c897fb308ee
Reviewed-on: https://code.wireshark.org/review/4592
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>