Remove a fossil from an ol d port hack.

Some driver files had the following code snuppet in them:

extern int async_write(int, const void *, unsigned int);

Specifically, refclock_acts.c, refclock_arbiter.c, refclock_dumbclock.c,
refclock_hopfser.c, refclock_nmea.c, refclock_palisade.c, refclock_true.c.

However, many more drivers use write(2) without redefining it, and all
run synchronously. I conclude that this snippet is unecessary and was
copied blindly from an early example.  Perhaps it was a performance hack.