ifnet: Integrate if_unroute() into if_down()

if_unroute() is static since 2004 and is not used anywhere except for
if_down().

This also makes it easier to grep by the pattern if_flags &= ~IFF_UP.

No functional change intended.

Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49356

(cherry picked from commit 0693c3f7cb3d9e5389863a0c131ddef799c77091)