Remove DUMMYNET from FreeNAS kernel as unused, keep module.

In FreeBSD 9.x DUMMYNET forces up to two CPUs of idle machine generate
1000 interrupts per second.  Avoiding that reduces idle CPU usage in
half according to VMWare accounting -- from 136MHz to 56MHz.

(cherry picked from commit d1c5557a6ede53c573882211d57c1c3472013f71)