pf: remove fastroute tag

The tag fastroute came from ipf and was removed in OpenBSD in 2011. The code
allows to skip the in pfil hooks and completely removes the out pfil invoke,
albeit looking up a route that the IP stack will likely find on its own.
The code between IPv4 and IPv6 is also inconsistent and marked as "XXX"
for years.

Submitted by: Franco Fichtner <franco@opnsense.org>
Differential Revision: https://reviews.freebsd.org/D8058