Remove proto_tree_add_text API.

Remove proto_tree_add_text API.

Its time has finally come.

Technically I just renamed it to proto_tree_add_text_internal and removed the WS_DLL_PUBLIC (so it shouldn't link outside of epan).  It's still (legitimately) used by expert.c otherwise I would have made it static within proto.c (and the rename wouldn't have been necessary).

Change-Id: I9bdf888d5e92bc7b70a3f5461b9297a66d994b80
Reviewed-on: https://code.wireshark.org/review/10594
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>