bug#12450: Remove configure's --without-sync-input option.

[Eli Zaretskii]

> Date: Sat, 22 Sep 2012 02:34:58 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 12450@debbugs.gnu.org
> 
> When they use the word "asynchronously", I assume
> they're talking about from the Lisp point of view,
> not the Unix signal-handler point of view.

The comments in redisplay code don't talk about Lisp.  There's no Lisp
running when redisplay happens, except through safe_eval.  These
comments talk about asynchronous entry to redisplay mentioned in the
large commentary near the beginning of xdisp.c.  AFAIU, this
asynchronous entry is no longer possible.