bug#26180: Remove motif toolkit support

[Glenn Morris]

Package: emacs
Version: 25.2
Severity: wishlist

I'd like to suggest removing support for the Motif toolkit.

Motivation:
Supporting more toolkits (gtk2, gtk3, lucid, motif, none) makes it
harder to develop relevant low-level features.

The Motif toolkit build of Emacs seems to be very little used:
  - Fedora packages just a Gtk version
  - Debian packages Gtk and Lucid
  - Only ~20 mentions of "motif" on bug-gnu-emacs, ever. Most are just
  comparison testing

(I see https://wiki.gentoo.org/wiki/GNU_Emacs mentions it, but my
suspicion is that this is just an attempt at completeness when listing
alternatives to Gtk. Since it's listed first, I wonder if this has
caused more people to use it than otherwise would. All just speculation
on my part.)

(The "none" toolkit is also never used in practice, but I understand it
may be useful as a base/reference.)