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

[Richard Stallman]

I have a vague memory that the sync input code had an important flaw
-- that for certain display-updating tasks (perhaps
mouse-highlighting) it could cause a delay until Emacs was read to
accept input, which the other code could do instantly.

I don't know if this is true now, and I'm not sure of the memory at all.
But it would be good to investigate this question.
Maybe there is a way to fix the problem.

-- 
Dr Richard Stallman
President, Free Software Foundation
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