pfctl: FreeBSD now supports DIOCGIFSPEED

There's no need for a special case here to work around the lack of
DIOCGIFSPEED. That was introduced in FreeBSD in
c1aedfcbd9896401f637bc815ba4e51dca107f6f.

Reported by: jmg@
Reviewed by: donner@
Differential Revision: https://reviews.freebsd.org/D28305