[NET]: Remove dead net/sched/Makefile entry for sch_hpfq.o.

Remove the worthless net/sched/Makefile entry for the non-existent
source file sch_hpfq.c.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: David S. Miller <davem@davemloft.net>