mapi: THREADS was always defined, remove it

mapi: THREADS was always defined, remove it

THREADS was defined if HAVE_PTHREADS or _WIN32 was defined.  That's
always the case.  The build would die in c11/threads.h otherwise.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>