REORG: include: move MAX_THREADS to defaults.h

REORG: include: move MAX_THREADS to defaults.h

That's already where MAX_PROCS is set, and we already handle the case of
the default value so there is no reason for placing it in thread.h given
that most call places don't need the rest of the threads definitions. The
include was removed from global-t.h and activity.c.