Unused synth_immediate method

[Samuel Thibault]

Okash Khawaja, on lun. 20 févr. 2017 21:49:31 +0000, wrote:
> Keeping spk_ttio_out() as it is also means that it mirrors
> spk_serial_out behaviour more closely while having few side effects,
> i.e. restarting of TTYs.

Actually, spk_serial_out does have the side effect of restarting TTYs
etc. on error, that's done by the spk_wait_for_xmitr() call. That's
actually one of the reasons why I was thinking that spk_ttyio_out should
be doing it.

Samuel