Eliminate proto_tree_add_text from packet-cdp.c

Change-Id: I77e1f5788e0567dcaef9f75e1f45e61c63d54b4e
Reviewed-on: https://code.wireshark.org/review/4849
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>