bug#16702: 24.3.50; flyspell in message mode messes up latin prefix input

[Stefan Monnier]

> Of course, I'm not saying we should actually revert this change: it
> was done for a reason.

The reason was another interaction with the same sit-for in flyspell,
where the event read needed to be read while inheriting input method.

We fundamentally have two bugs:
- sit-for shouldn't use read-event: as noted it's impossible to
  undo it via unread-command-events.
- flyspell shouldn't use sit-for: it should return immediately and start
  a timer to do the delayed check.


        Stefan