route.8: remove now incorrect bug report about blackhole routes

We removed IP fast forwarding a while ago, so this is no longer relevant.
The netlink interface always sets the nexthop interface as V_loif, so users
don't need to worry about it either.

Reported by: Jim Thompson
Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47528