bug#50444: Remove support for the curses library

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Tue, 7 Sep 2021 02:28:31 +0200
> Cc: paul eggert <eggert@cs.ucla.edu>
> 
> Building with curses is, according to Paul Eggert:
> 
>     "Completely broken" in the sense it won't link and Emacs won't build,
>     yes. (It will link in master, due to a patch I installed recently.)
> 
>     And yes, the original curses library has been obsolete for at least two
>     decades.
> 
> See the discussion here:
> https://lists.gnu.org/archive/html/emacs-devel/2021-01/msg00416.html
> 
> The conclusion from that thread AFAIU is that support for curses
> should be removed.  (We should keep ncurses though.)

Doesn't that mean we will be removing support for AIX (and any other
systems that cannot use ncurses due to the same problem)?  Are we
willing to drop support for those platforms because of curses?  AFAIU,
the original problem was fixed, and there's no longer a link error
when using curses.

In any case, if we are planning to drop support for curses, we need
to obsolete it first, i.e. add a configure-time warning for that.