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

[Eli Zaretskii]

> Date: Sun, 16 Sep 2012 14:48:55 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: rms@gnu.org, lekktu@gmail.com, 12450-done@debbugs.gnu.org
> 
> Since the patch is installed now I'm marking this bug as done.
> If bugs crop up we can reopen it or open a new bug report.

There are still a lot of comments in the code that describe the
non-SYNC_INPUT mode.  For example, see blockinput.h.