ifnet(9): Remove mention of defunct ifaddr_byindex(9).

Fixes: 964b8f8b993d
MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46174

(cherry picked from commit 9f080f17a6a740ce9c1fa45862fb689263908c90)