Rip out the filetap code

We decided at sharkfest that this wasn't the right design for file dissection;
we have more-or-less settled on way forward, but nobody's shown interest in
implementing it. Whether or not that ever happens, this code is effectively
dead and should be removed.

Change-Id: I14d6086df3204fffb6485228db39d9f407661417
Reviewed-on: https://code.wireshark.org/review/3400
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>