Running TTS and encoders with multiple threads is causing problems on Windows
since introduction of the feature (FS#12106, FS#11994). The current
implementation also makes wrong assumptions (having multiple threads talk to
the SAPI script doesn't make it run faster since it's still one thread
responsible for creation).
Completely remove multithreading support for that for now -- a different
implementation is necessary.
Change-Id: Icafa223644efc370a09186ce28ac83c22902e0c0
820dcfdfed